Downloaded from equations. 2.4 The reciprocal function x 1 x


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1 Functions and equations Assessment statements. Concept of function f : f (); domain, range, image (value). Composite functions (f g); identit function. Inverse function f.. The graph of a function; its equation f (). Function graphing skills: use of a GDC to graph a variet of functions; investigation of ke features of graphs. s of equations graphicall.. Transformations of graphs: translations; stretches; reflections in the aes. The graph of f () as the reflection in the line of the graph f ().. The reciprocal function, 0: its graph; its selfinverse nature..5 The quadratic function a b c: its graph, intercept (0, c), ais of smmetr _ b a. The form a( h) k: verte (h,k). The form a( p)( q): intercepts (p, 0) and (q, 0)..6 The solution of a b c 0, a 0. The quadratic formula. Use of the discriminant b ac. Introduction This chapter looks at functions and considers how the can be used in describing phsical phenomena. We also investigate composite and inverse functions, and transformations such as translations, stretches and reflections. Quadratic functions are treated graphicall and algebraicall. Most countries ecept the United States use the Celsius scale, invented b the Swedish scientist Anders Celsius (707). The United States uses the earlier Fahrenheit scale, invented b the Dutch scientist Gabriel Daniel Fahrenheit (68676). A citizen of the USA travelling to other parts of the world will need to convert from degrees Celsius to degrees Fahrenheit.. Relations and functions Relations There are different scales for measuring temperature. Two of the more commonl used are the Celsius scale and the Fahrenheit scale. A temperature recorded in one scale can be converted to a value in the other scale, based on the fact that there is a constant relationship between the two sets of numbers in each scale. If the variable C represents degrees Celsius and the variable F represents degrees Fahrenheit, this relationship can be epressed b the following equation that converts Celsius to Fahrenheit: F 9_ 5 C.
2 Man mathematical relationships concern how two sets of numbers relate to one another and often the best wa to epress this is with an algebraic equation in two variables. If it s not too difficult, we find it useful to epress one variable in terms of the other. For eample, in the previous equation, F is written in terms of C making C the independent variable and F the dependent variable. Since F is written in terms of C, it is easiest for ou to first substitute in a value for C, and then evaluate the epression to determine the value of F. In other words, the value of F is dependent upon the value of C that is chosen independent of F. A relation is a rule that determines how a value of the independent variable corresponds or is mapped to a value of the dependent variable. A temperature of 0 degrees Celsius corresponds to 86 degrees Fahrenheit. F 9_ 5 (0) 5 86 Along with equations, other useful was of representing a relation include a gr aph of the equation on a Cartesian coordinate sstem (also called a rectangular coordinate sstem), a table, a set of ordered pairs, or a mapping. These are illustrated below for the equation F 9_ 5 C. Graph F C Table Celsius (C) Fahrenheit (F) René Descartes The Cartesian coordinate sstem is named in honour of the French mathematician and philosopher René Descartes ( ). Descartes stimulated a revolution in the stud of mathematics b merging its two major fields algebra and geometr. With his coordinate sstem utilizing ordered pairs (Cartesian coordinates) of real numbers, geometric concepts could be formulated analticall and algebraic concepts (e.g. relationships between two variables) could be viewed graphicall. Descartes initiated something that is ver helpful to all students of mathematics that is, considering mathematical concepts from multiple perspectives: graphical (visual) and analtical (algebraic). Ordered pairs The graph of the equation F 9_ 5 C is a line consisting of an infinite set of ordered pairs (C, F) each is a solution of the equation. The following set includes some of the ordered pairs on the line: {( 0, ), (0, ), (0, 68), (0, 0)}. Mapping Domain (input) Range (output) 0 0 C F Rule: F C 5 Hint: The coordinate sstem for the graph of an equation has the independent variable on the horizontal ais and the dependent variable on the vertical ais. The largest possible set of values for the independent variable (the input set) is called the domain and the set of resulting values for the dependent variable (the output set) is called the range. In the contet of a mapping, each value in the domain is mapped to its image in the range.
3 Functions and equations Downloaded from Functions If the relation is such that each number (or element) in the domain produces one and onl one number in the range, the relation is called a function. Common sense tells us that each numerical temperature in degrees Celsius (C) will convert (or correspond) to onl one temperature in degrees Fahrenheit (F). Therefore, the relation given b the equation F 9 _ 5 C is a function  an chosen value of C corresponds to eactl one value of F. The idea that a function is a rule that assigns to each number in the domain a unique number in the range is formall defined below. Definition of a function A function is a correspondence (mapping) between two sets X and Y in which each element of set X corresponds to (maps to) eactl one element of set Y. The domain is set X (independent variable) and the range is set Y (dependent variable). Not onl are functions important in the stud of mathematics and science, we encounter and use them routinel often in the form of tables. Eamples include height and weight charts, income ta tables, loan pament schedules, and time and temperature charts. The importance of functions in mathematics is evident from the man functions that are installed on our GDC. For eample, the kes labelled SIN LN In(0) each represent a function, because for each input (entr) there is onl one output (answer). The calculator screen image shows that for the function n, the input of 0 has onl one output of For man phsical phenomena, we observe that one quantit depends on another. For eample, the boiling point of water depends on elevation above sea level; the time for a pendulum to swing through one ccle (its period) depends on the length of the pendulum; and the area of a circle depends on its radius. The word function is used to describe this dependence of one quantit on another i.e. how the value of an independent variable determines the value of a dependent variable. Boiling point is a function of elevation (elevation determines boiling point). The period of a pendulum is a function of its length (length determines period). The area of a circle is a function of its radius (radius determines area). Eample a) Epress the volume V of a cube as a function of the length e of each edge. b) Epress the volume V of a cube as a function of its surface area S. e e e
4 a) V as a function of e is V e. b) The surface area of the cube consists of si squares each with an area of e. Hence, the surface area is 6e ; that is, S 6e. We need to write V in terms of S. We can do this b first epressing e in terms of S, and then substituting this epression in for e in the equation V e. S 6e e S e 6 S 6. Substituting, 6 ) ( S _ ) S S S S ( 6 _ ) 6 _ 6 6 _ V as a function of S is V S 6 S 6. V ( S 6 S 6. Domain and range of a function The domain of a function ma be stated eplicitl, or it ma be implied b the epression that defines the function. If not eplicitl stated, the domain of a function is the set of all real numbers for which the epression is defined as a real number. For eample, if a certain value of is substituted into the algebraic epression defining a function and it causes division b zero or the square root of a negative number (both undefined in the real numbers) to occur, that value of cannot be in the domain. The domain of a function ma also be implied b the phsical contet or limitations that eist. Usuall the range of a function is not given eplicitl and is determined b analzing the output of the function for all values of the input. The range of a function is often more difficult to find than the domain, and analzing the graph of a function is ver helpful in determining it. A combination of algebraic and graphical analsis is ver useful in determining the domain and range of a function. Eample Find the domain of each of the following functions. a) {( 6, ), (, 0), (, ), (, 0), (5, )} b) Area of a circle: A r c) d) a) The function consists of a set of ordered pairs. The domain of the function consists of all first coordinates of the ordered pairs. Therefore, the domain is the set { 6,,,, 5}. b) The phsical contet tells ou that a circle cannot have a negative radius. You can onl choose values for the radius (r) that are greater than or equal to zero. Therefore, the domain is the set of all real numbers such that r 0. 5
5 Functions and equations Downloaded from c) The value of 0 cannot be included in the domain because division b zero is not defined for real numbers. Therefore, the domain is the set of all real numbers ecept zero ( 0). d) An negative values of cannot be in the domain because the square root of a negative number is not a real number. Therefore, the domain is all real numbers such that 0. 0 Figure. Determining if a relation is a function Some relations are not functions and because of the mathematical significance of functions it is important for us to be able to determine when a relation is, or is not, a function. It follows from the definition of a function that a relation for which a value of the domain () corresponds to (or determines) more than one value in the range () is not a function. An two points (ordered pairs (, )) on a vertical line have the same coordinate. Although a trivial case, it is useful to recognize that the equation for a vertical line, for eample (see Figure.), is a relation but not a function. The points with coordinates (, ), (, 0) and (, ) are all solutions to the equation. The number two is the onl element in the domain of but it is mapped to more than one value in the range (, 0 and, for eample). It follows that if a vertical line intersects the graph of a relation at more than one point, then a value in the domain () corresponds to more than one value in the range () and, hence, the relation is not a function. This argument provides an alternative definition of a function and also a convenient visual test to determine whether or not the graph of a relation represents a function. Alternative definition of a function A function is a relation in which no two different ordered pairs have the same first coordinate. Vertical line test for functions A vertical line intersects the graph of a function at no more than one point. Figure. As the graph in Figure. clearl shows, a vertical line will intersect the graph of at no more than one point therefore, the relation is a function. 0 5 Rule: Domain (input) Range (output) Each element of the domain () is mapped to eactl one element of the range (). 6
6 In contrast, the graph of the equation is a sidewas parabola that can clearl be intersected more than once b a vertical line (see Figure.). There are at least two ordered pairs having the same coordinate but different coordinates (for eample, (9, ) and (9, )). Therefore, the relation fails the vertical line test indicating that it does not represent a function. 0 Rule: or Domain (input) Range (output) Figure At least one element of the domain () is mapped to more than one element of the range (). Hint: Note that to graph the equation on our _ GDC, ou need _ to solve for _ in terms of. The result is two separate equations: and, (or ). Each is onehalf of the sidewas parabola. Although each represents a function (vertical line test), the combination of the two is a complete graph of that clearl does not satisf either definition of a function. Y= ( X) X=9 Y= Y= ( X) X=9 Y= Plot Plot Y= (X) Y=  (X) Y= Y= Y5= Y6= Y7= Plot Eample What is the domain and range for the function? Algebraic analsis: Squaring an real number produces another real number. Therefore, the domain of is the set of all real numbers ( ). What about the range? Since the square of an positive or negative number will be positive and the square of zero is zero, the range is the set of all real numbers greater than or equal to zero. Graphical analsis: For the domain, focus on the ais and horizontall scan the graph from to. There are no gaps or blank regions in the graph and the parabola will continue to get wider as goes to either or. Therefore, the domain is all real numbers. For the range, focus on the ais and vertical scan from or. The parabola will continue higher as goes to, but the graph does not go below the ais. The parabola has no points with negative coordinates. Therefore, the range is the set of real numbers greater than or equal to zero. See Figure domain Figure. range 7
7 Functions and equations Downloaded from Table. Different was of epressing the domain and range of. Hint: The infinit smbol does not represent a number. When or is used in interval notation, it is being used as a convenient notational device to indicate that an interval has no endpoint in a certain direction. Hint: When asked to determine the domain and range of a function, it is wise for ou to conduct both algebraic and graphical analsis and not rel too much on either approach. For graphical analsis of a function, producing a comprehensive graph on our GDC is essential and an essential skill for this course. Description in words domain is an real number range is an real number greater than or equal to zero Function notation Interval notation (both formats) domain is { : } or domain is ], [ range is { : 0} or range is [0, [ It is common practice to assign a name to a function usuall a single letter with f, g and h being the most common. Given that the domain (independent) variable is and the range (dependent) variable is, the smbol f (), read f of, denotes the unique value of that is generated b the value of. This function notation was devised b the famous Swiss mathematician Leonhard Euler (70778). Another notation sometimes referred to as mapping notation is based on the idea that the function f is the rule that maps to f () and is written f : f (). For each value of in the domain, the corresponding unique value of in the range is called the function value at, or the image of under f. The image of ma be written as f () or as. For eample, for the function f () : f () 9 ; or, if then 9. Table. Function notation. Notation Description in words f () the function f, in terms of, is ; or, simpl f of is f : the function f maps to f () 9 the value of the function f when is 9 ; or, simpl f of equals 9 f : 9 the image of under the function f is 9 h() 8 Eample Find the domain and range of the function h :. Algebraic analsis: The function produces a real number for all, ecept for when division b zero occurs. Hence, is the onl real number not in the domain. Since the numerator of can never be zero, the value of cannot be zero. Hence, 0 is the onl real number not in the range. Graphical analsis: A horizontal scan shows a gap at dividing the graph of the equation into two branches that both continue indefinitel with no other gaps as. Both branches are asmptotic (approach but do not intersect) to the vertical line. This line is a vertical asmptote and is drawn as a dashed line (it is not part of the graph of the equation). A vertical scan reveals a gap at 0 (ais)
8 with both branches of the graph continuing indefinitel with no other gaps as. Both branches are also asmptotic to the ais. The ais is a horizontal asmptote. Both approaches confirm the following for h : : The domain is { :, } or ], [ ], [ The range is { :, 0} or ], 0[ ]0, [ Eample 5 Consider the function g (). a) Find: (i) g (7) (ii) g () (iii) g ( ) b) Find the values of for which g is undefined. c) State the domain and range of g. g() _ a) (i) g (7) 7. ( significant figures) (ii) g () 6 6 _ (iii) g ( ) 0 0 b) g () will be undefined (square root of a negative) when Therefore, g () is undefined when. c) It follows from the result in b) that the domain of g is { : }. The smbol stands for the principal square root that, b definition, can onl give a result that is positive or zero. Therefore, the range of g is { : 0}. The domain and range are confirmed b analzing the graph of the function. Eample 6 Find the domain and range of the function f () 9. Y=/ ( 9X) The graph of on a GDC, shown 9 X=0 Y=. right, agrees with algebraic analsis indicating that the epression will be positive for all, and is defined onl for. 9 Further analsis and tracing the graph reveals that f () has a minimum at ( 0, _ ). The graph on the GDC is misleading in that it appears to show that the function has a maimum value of approimatel Can this be correct? A lack of algebraic thinking and overreliance on our GDC could easil lead to a mistake. The graph abruptl stops its curve upwards because of low screen resolution. Function values should get quite Hint: As Eample 6 illustrates, it is dangerous to completel trust graphs produced on a GDC without also doing some algebraic thinking. It is important to mentall check that the graph shown is comprehensive (shows all important features of the graph), and that the graph agrees with algebraic analsis of the function e.g. where should the function be zero, positive, negative, undefined, increasing/ decreasing without bound, etc. 9 9
9 Functions and equations Downloaded from Y=/ ( 9X) X=.9787 X X=.999 Y= TABLE SETUP TblStart=.999 Tbl=.000 Indpnt: Depend: Ask Ask Y Y (.99999) Y( ) Y( ) ERROR Auto Auto large for values of a little less than, because the value _ of 9 will be small making the fraction 9 large. Using our GDC to make a table for f (), or evaluating the function for values of ver close to or, confirms that as approaches or, increases without bound, i.e. goes to. Hence, f () has vertical asmptotes of and. This combination of graphical and algebraic analsis leads to the conclusion that the domain of f () is { : }, and the range of f () is { : _ }. Eercise. For each equation 9, a) match it with its graph (choices are labelled A to L), and b) state whether or not the equation represents a function with a justification. Assume that is the independent variable and is the dependent variable A B C D E F G H I J K L 0
10 0 Epress the area, A, of a circle as a function of its circumference, C. Epress the area, A, of an equilateral triangle as a function of the length,, of each of its sides. In questions 7, find the domain of the function. f () _ 5 7 h() g (t) t 5 h(t) 6 Volume of a sphere: V _ r 7 g (k) _ 6 K 9 8 Do all linear equations represent a function? Eplain. 9 Find the domain and range of the function f defined as f : 5. 0 Consider the function h(). a) Find: (i) h() (ii) h(5) (iii) h() b) Find the values of for which h is undefined. c) State the domain and range of h. d) Sketch a comprehensive graph of the function. Find the domain and range of the function f defined as f () _ and sketch 9 a comprehensive graph of the function clearl indicating an intercepts or asmptotes. _ t. Composite of functions Composite functions Consider the function in Eample 5 in the previous section, f (). When ou evaluate f () for a certain value of in the domain (for eample, 5) it is necessar for ou to perform computations in two separate steps in a certain order. f (s) 5 f (5) 9 Step : compute the sum of 5 f (5) Step : compute the square root of 9 Given that the function has two separate evaluation steps, f () can be seen as a combination of two simpler functions that are performed in a specified order. According to how f () is evaluated (as shown above), the simpler function to be performed first is the rule of adding and the second is the rule of taking the square root. If h() and g (), we can create (compose) the function f () from a combination of h() and g () as follows: f () g(h()) g( ) Step : substitute for h() making the argument of g() Step : appl the function g() on the argument We obtain the rule b first appling the rule and then appling the rule. A function that is obtained from simpler functions b appling one after another in this wa is called a composite function. In the eample above, f () is the composition of h() From the eplanation on how f is the composition (or composite) of g and h, ou can see wh a composite function is sometimes referred to as a function of a function. Also note that in the notation g(h()), the function h that is applied first is written inside, and the function g that is applied second is written outside.
11 Functions and equations Downloaded from Figure.5 followed b g(). In other words, f is obtained b substituting h into g, and can be denoted in function notation b g(h()) read g of h of. g h h g h() g(h()) domain of h range of h domain of g range of g We start with a number in the domain of h and find its image h(). If this number h() is in the domain of g, we then compute the value of g (h()). The resulting composite function is denoted as (g h)(). See mapping illustration in Figure.5. Definition of the composition of two functions The composition of two functions, g and h, such that h is applied first and g second is given b (g h)() g (h()) The domain of the composite function g h is the set of all in the domain of h such that h() is in the domain of g. Hint: The notations (g h)() and g (h()) are both commonl used to denote a composite function where h is applied first then followed b appling g. Since we are reading this from left to right, it is eas to appl the functions in the incorrect order. It ma be helpful to read g h as g following h, or as g composed with h to emphasize the order in which the functions are applied. Also, in either notation, (g h)() or g (h()), the function applied first is closest to the variable. Eample 7 If f () and g () 6, find: a) (f g )(5) b) Epress (f g )() as a single function rule (epression). c) (g f )(5) d) Epress (g f )() as a single function rule (epression). e) (g g )(5) f) Epress (g g )() as a single function rule (epression). a) (f g)(5) f (g (5)) f ( 5 6) f () b) (f g)(5) f (g ()) f ( 6) ( 6) 6 8 Therefore, (f g)() 6 8 Check with result from a): (f g)(5) c) (g f )(5) g (f (5)) g ( 5) g (5) 5 6 d) (g f )() g (f ()) g () () Therefore, (g f )() 6 6 Check with result from c): (g f )(5) e) (g g)(5) g (g (5)) g ( 5 6) g () 6 f) (g g)() g (g ()) g ( 6) ( 6) 6 8 Therefore, (g g)() 8 Check with result from e): (g g)(5)
12 It is important to notice that in parts b) and d) in Eample 7, f g is not equal to g f. At the start of this section, it was shown how the two functions h() and g () could be combined into the composite function (g h)() to create the single function f (). However, the composite function (h g)() the functions applied in reverse order creates a different function: (h g)() h (g ()) h( ). Since, then again f g is not equal to g f. Is it alwas true that f g g f? The net eample will answer that question. Eample 8 Given f : 6 and g : _, find the following: a) (f g)() b) (g f )() a) (f g)() f (g ()) f ( _ ) ( _ ) b) (g f )() g (f ()) g ( 6) _ ( 6) Eample 8 shows that it is possible for f g to be equal to g f. We will learn in the net section that this occurs in some cases where there is a special relationship between the pair of functions. However, in general f g g f. Decomposing composite functions In Eamples 7 and 8, we created a single function b forming the composition of two functions. As we did with the function f () at the start of this section, it is also important for ou to be able to identif two functions that make up a composite function, in other words, for ou to decompose a function into two simpler functions. When ou are doing this it is ver useful to think of the function which is applied first as the inside function, and the function that is applied second as the outside function. In the function f (), the inside function is h() and the outside function is g(). Hint: Decomposing composite functions identifing the component functions that form a composite function is an important skill when working with certain functions in the topic of calculus. For the composite function f () (g h)(), g and h are the component functions. Eample 9 Each of the following functions is a composite function of the form (f g)(). For each, find the two component functions f and g. a) h : b) k : c) p() _ a) If ou were to evaluate the function h() for a certain in the domain, ou would first evaluate the epression, and then evaluate the epression. Hence, the inside function (applied first) is, and the outside function (applied second) is. Then, with g () and f (), it follows that h : (f g)().
13 Functions and equations Downloaded from b) Evaluating k () requires ou to first evaluate the epression, and then evaluate the epression. Hence, the inside function is, and the outside function is. Then, with g () and f (), it follows that k : (f g)(). c) Evaluating p() requires ou to perform three separate evaluation steps : () squaring a number, () subtracting four, and then () taking the cube root. Hence, it is possible to decompose p() into three component functions: if h(), g () and f (), then p() (f g h)() f (g(h())). However, for our purposes it is best to decompose the composite function into onl two component functions: if g(), and f (), then p : (f g)() f (g()). domain of h Figure.6 h g h h() range of h domain of g g g(h()) range of g Finding the domain of a composition of functions Referring back to Figure.5 (shown again here as Figure.6), it is important to note that in order for a value of to be in the domain of the composite function g h, two conditions must be met: () must be in the domain of h, and () h() must be in the domain of g. Likewise, it is also worth noting that g (h()) is in the range of g h onl if is in the domain of g h. The net eample illustrates these points and also that, in general, the domains of g h and h g are not the same. Eample 0 Let g () and h(). Find: a) (g h)() and its domain and range, and b) (h g)() and its domain and range. Firstl, establish the domain and range for both g and h. For g (), the domain is and the range is. For h(), the domain is 0 and the range is 0. a) (g h)() g (h()) g ( ) To be in the domain of g h, must be defined for 0. ( ) Therefore, the domain of g h is 0. Since 0, the range for is. Therefore, (g h)(), and its domain is 0, and its range is. b) (h g)() h (g()) g () must be in the domain of h h 0. h ( ) Therefore, the domain of h g is or _ and with or, the range for _ is 0. _ Therefore, (h g)(), and its domain is or, and its range is 0.
14 Eercise. Let f () and g(), 0. a) Find the value of (i) (f g)(5) and (ii) (g f )(5). b) Find the function rule (epression) for (i) (f g)() and (ii) (g f )(). Let f : and g :. In af, evaluate: a) (f g)(0) b) (g f )(0) c) (f f )() d) (g g)( ) e) (f g)( ) f ) (g f )( ) In gj, find the epression: g) (f g)() h) (g f )() i) (f f )() j) (g g)() For each pair of functions in 7, find (f g)() and (g f )() and state the domain for each. f (), g() f (), g() 5 f (), g() 6 f (), g() 7 f () 5, g() 5 8 Let g() and h() 0. Find: a) (g h)() and its domain and range b) (h g)() and its domain and range. In 9, determine functions g and h so that f () g(h()). 9 f () ( ) 0 f () 5 f () 7 f () f () 0 f () 9 In 58, find the domain for a) the function f, b) the function g, and c) the composite function f g. 5 f (), g() 6 f (), g() 7 f () _, g() 8 f (), g(). Inverse functions Pairs of inverse functions Let s look again at the function at the start of this chapter the formula that converts degrees Celsius (C) to degrees Fahrenheit (F): F 9 5 C. If we rearrange the function so that C is the independent variable (i.e. C is epressed in terms of F ), we get a different formula that does the reverse, or inverse process, andconverts F to C. Writing C in terms of F (solving for C) gives: C 5 9 (F )or C 5 9 F _ This new formula could be useful for people travelling to the USA. These two conversion formulas, F 9 5 C and C 5 9 F _ 60 9, are both 5
15 Functions and equations Downloaded from Hint: Writing a function using and for the independent and dependent variables, such that is epressed in terms of, is a good idea because this is the format in which ou must enter it on our GDC in order to have the GDC displa a graph or table for the function. Plot domain of f f C g range of f 5 77 range of g Figure.7 Plot Plot Y=(9/5)X+ Y=(5/9)X60/9 Y= Y= Y5= Y6= F domain of g linear functions. As mentioned previousl, it is tpical for the independent variable (domain) of a function to be and the dependent variable (range) to be. Let s assign the name f to the function converting C to F, and the name g to the function converting F to C. converting C to F: 9 5 f () 9 5 converting C to F: 5 _ g () 5 _ The two functions, f and g, have a special relationship in that the undo each other. To illustrate, function f converts 5 C to 77 F [ f (5) 9 5 (5) 5 77 ],and then function g can undo this b converting 77 F back to 5 C [ g (77) 5 9 (77) _ this reverse (inverse) effect on function f, we call function g the inverse of function f. Function f has the same inverse effect on function g [g(77) 5 and then f (5) 77], making f the inverse function of g. The functions f and g are inverses of each other the are a pair of inverse functions. _ ]. Because function g has In Figure.7, the mapping diagram for the functions f and g illustrates the inverse relationship for a pair of inverse functions where the domain of one is the range for the other. You are alread familiar with pairs of inverse operations. Addition and subtraction are inverse operations. For eample, the rule of adding si ( 6), and the rule of subtracting si ( 6) undo each other. Accordingl, the functions f () 6 and g () 6 are a pair of inverse functions. Multiplication and division are also inverse operations. 6 The composition of two inverse functions The mapping diagram (Figure.7) and the numerical eamples in the previous paragraph indicate that if function f is applied to a number in its domain (e.g. 5) giving a result in the range of f (i.e. 77) and then function g is applied to this result, the final result (i.e. 5) is the same number first chosen from the domain of f. This process and result can be epressed smbolicall as: (g f )() or g (f ()). The composition of two inverse functions maps an value back to itself i.e. one function undoing the other. It must also follow that (f g). Let s verif these results for the pair of inverse functions f and g. 5 ) _ 60 _ 60 _ ( 5 _ ) _ 60 5 (g f )() g ( 9 5 ) 5 9 ( 9 f (g()) f ( _ 9 ) 9 Eamples 7 and 8 in the previous section on composite functions eplored whether f g g f. Eample 7 provided a counter eample showing it is not a true statement. However, Eample 8 showed a pair of functions for which (f g)() (g f )() ; the same result that we just obtained for the pair of inverse functions that convert between C and F. The two functions in Eample 8, f : 6 and g : _, are also a pair of inverse functions.
16 Definition of the inverse of a function If f and g are two functions such that (f g)() for ever in the domain of g and (g f )() for ever in the domain of f, the function g is the inverse of the function f. The notation to indicate the function that is the inverse of function f is f. Therefore, (f f )() and (f f )() The domain of f must be equal to the range of f, and the range of f must be equal to the domain of f. Figure.7 shows a mapping diagram for a pair of inverse functions. Finding the inverse of a function Eample Given the linear function f () 8, find its inverse function f () and verif the result b showing that (f f )() and (f f )(). Recall that the wa we found the inverse of the function converting C to F, F 9_ 5 C, was b making the independent variable the dependent variable and vice versa. Essentiall what we are doing is switching the domain () and range () since the domain of f becomes the range of f and the range of f becomes the domain of f, as stated in the definition of the inverse of the function, and depicted in Figure.8. Also, recall that f (). f () 8 8 write f () 8 interchange and (i.e. switch the domain and range) 8 solve for (dependent variable) in terms of (in dependent variable) _ f () _ resulting equation is f () Verif that f and f are inverses b showing that f (f ()) and f (f ()). f ( _ ) ( _ ) f ( 8) _ ( 8) This confirms that 8 and _ are inverses of each other. The method of interchanging and to find the inverse function also gives us a wa for obtaining the graph of f from the graph of f. Given the reversing effect that a pair of inverse functions have on each other, if f (a) b then f (b) a. Hence, if the ordered pair (a, b) is a point on the graph of f (), the reversed ordered pair (b, a) must be on the graph of f (). Figure.9 shows that the point (b, a) can be found b reflecting the point (a, b) about the line. As Figure.0 illustrates, the following is true. Graphical smmetr of inverse functions The graph of f is a reflection of the graph of f about the line. domain of f range of f f f range of f domain of f Figure.8 f () and f () It follows from the definition that if g is the inverse of f, it must also be true that f is the inverse of g. Hint: Do not mistake the in the notation f for an eponent. It is not an eponent. f does not denote the reciprocal of f (). If a superscript of is applied to the name of a function as in f () or sin () it denotes the function that is the inverse of the named function (e.g. f () or sin()). If a superscript of is applied to an epression, as in 7 or ( 5) or (f ()), it is an eponent and denotes the reciprocal of the epression. For eample, the reciprocal of f () is (f ()) _ f (). (b, a) Figure.9 Figure.0 f (a, b) f 7
17 Functions and equations Downloaded from The identit function We have repeatedl demonstrated the fact, and it is formall stated in the definition of the inverse of a function, that the composite function which has a pair of inverse functions as its components is alwas the linear function. That is, (f f )() or (f f )(). Let s label the function with the name I. Along with the fact that I() (f f )() (f f )(), the function I() has two other interesting properties. It is obvious that the line is reflected back to itself when reflected about the line. Hence, from the graphical smmetr of inverse functions, the function I() is its own inverse; that is, I() I (). Most interestingl, I() behaves in composite functions just like the number one behaves for real numbers and multiplication. The number one is the identit element for multiplication. For an function f, it is true that f I f and I f f. For this reason, we call the function f (), or I(), the identit function. When f () f () the function f is said to be selfinverse. The fact that the function f () is selfinverse should make ou wonder if there are an other functions with the same propert. Knowing that inverses are smmetric about the line, we onl need to find a function whose graph has as a line of smmetr. The eistence of an inverse function Is it possible for the inverse of a function not to be a function? Recall that the definition of a function (Section.) sas that a function is a relation such that a certain value in the domain produces onl one value in the range. The vertical line test for functions followed from this definition. Eample Find the inverse of the function g () with domain. Following the method used in Eample : g() Figure. Certainl the graphs of and are reflections about the line (see Figure.). However, the graph of does not pass the vertical line test. is the inverse of g (), but it is onl a relation and not a function. The inverse of g () will be a function onl if g () is a onetoone function; that is, a function such that no two elements in the domain () of g correspond to the same element in the range (). The graph of a onetoone function must pass both a vertical line test and also a horizontal line test. 8
18 The function f () with domain (Figure.) is not a onetoone function. Hence, its inverse is not a function. There are two different values of that correspond to the same value of ; for eample, and both get mapped to. Hence, f does not pass the horizontal line test Figure. The function f () with domain 0 is a onetoone function (Figure.). Hence, its inverse is also a function. [Note: domain changed to 0.] A function f has an inverse function f if and onl if f is onetoone Figure. Definition of a onetoone function A function is onetoone if each element in the range is the image of eactl one element in the domain. No horizontal line can pass through the graph of a onetoone function at more than one point (horizontal line test). Referring back to Eample, ou now understand that the function g () with domain does not have an inverse function g (). However, if the domain is changed so that g () is onetoone, then g () eists. There is not onl one wa to change the domain of a function in order to make it onetoone. Eample Given g () such that 0, find g () and state its domain. Given that the domain is 0, then the range for g () will be. Since the domain and range are switched for the inverse, for g () the domain is and the range is. Given the working in Eample, it follows that g () with domain., 0, 9
19 Functions and equations Downloaded from Eample Given g () such that, find g () and state its domain. Eaminer s Hint: For the Mathematics Standard Level course, if an inverse function is to be found, the given function will be defined with a domain that ensures it is onetoone. Given that the domain is, then the range for g () will be. Since the domain and range are switched for the inverse, for g () the domain is and the range is. Given the working in Eample, it follows that g () with domain. Finding the inverse of a function To find the inverse of a function f, use the following steps: Confirm that f is onetoone (although, for this course, ou can assume this). Replace f () with. Interchange and. Solve for. 5 Replace with f (). 6 The domain of f is equal to the range of f; and the range of f is equal to the domain of f. Eample 5 Consider the function f :,. a) Determine the inverse function f. b) What is the domain of f? a) Following the steps for finding the inverse of a function gives: replace f () with interchange and solve for ; squaring both sides here solved for f : replace with f () b) The domain eplicitl defined for f is and since the smbol stands for the principal square root (positive), then the range of f is all positive real numbers, i.e. 0. The domain of f is equal to the range of f, therefore, the domain of f is 0. Graphing and from Eample 5 on our GDC visuall confirms these results. Note that since the calculator would have automaticall assumed that the domain is, the domain for 50
20 the equation has been changed to. In order to show that f and f are reflections about the line, the line has been graphed and a viewing window has been selected to ensure that the scales are equal on each ais. Using the trace feature of our GDC, ou can eplore a characteristic of inverse functions that is, if some point (a, b) is on the graph of f, the point (b, a) must be on the graph of f. Plot Y= ( X+) Y=(X)(X > 0) Y= X Y= Y5= Y6= Y7= Plot Plot WINDOW Xmin=6 Xma=6 Xscl= Ymin= Yma= Yscl= Xres= Y=(X)(X>0) X= Y= Y= (X+) X= Y= Eample 6 Consider the function f () ( ) and g (). a) Find g and state its domain and range. b) Solve the equation (f g )() a) replace f () with interchange and solve for solved for g () replace with g () g is a linear function and its domain is and its range is ; therefore, for g the domain is and range is. b) (f g )() f (g ()) f ( ) [( ) ] _ Eample 7 Given f () 6, find the inverse f () and state its domain. SolutioAn The graph of f () 6,, is a parabola with a verte at (, 9). It is not a onetoone function. There are man was to restrict the domain of f to make it onetoone. The choices that have the domain as large as possible are or. Let s change the domain of f to. Y=X6X X= Y=9 5
21 Functions and equations Downloaded from 9 ( 9, ) Figure. (, 9) ( ) 9 9 replace f () with interchange and solve for b completing the square 9 rather than because range of f is (domain of f ) In order for 9 to be a real number then 9. Therefore, f () 9 and the domain of f is 9. The inverse relationship between f () 6 and f () 9 is confirmed graphicall in Figure.. Eercise. In questions, assume that f is a onetoone function. a) If f () 5, what is f ( 5)? b) If f (6) 0, what is f (0)? a) If f ( ), what is f ()? b) If f (b) a, what is f (a)? If g () 7, what is g (5)? If h () 8, with, what is f ( )? In questions 5, show a) algebraicall and b) graphicall that f and g are inverse functions b verifing that (f g)() and (g f )(), and b sketching the graphs of f and g on the same set of aes with equal scales on the  and aes. Use our GDC to assist in making our sketches on paper. 5 f : 6; g : 6 6 f : ; g : 7 f : 9; g : _ 8 f :, g : 9 f :, 0; g :, 0 f :, g : f : (6 ) _ ; g : 6, 0 f :, g : In questions 0, find the inverse function f and state its domain. f () f () 7 5 f () 6 f () 7 f (), 0 8 f () 5 9 f () a b, a 0 0 f (), In questions 8, use the functions g () and h () to find the indicated value or the indicated function. (g h )(5) (h g )(9) (g g )() (h h )() 5 g h 6 h g 7 (g h) 8 (h g) 9 The function in question 8, f (), is its own inverse (selfinverse). Show that an function in the form f () a b, a 0 is its own inverse. b 5
22 . Transformations of functions Even when ou use our GDC to sketch the graph of a function, it is helpful to know what to epect in terms of the location and shape of the graph and even more so if ou re not allowed to use our GDC for a particular question. In this section, we look at how certain changes to the equation of a function can affect, or transform, the location and shape of its graph. We will investigate three different tpes of transformations of functions that include how the graph of a function can be translated, reflected and stretched (or shrunk). This will give us a better understanding of how to efficientl sketch and visualize man different functions. Graphs of common functions It is important for ou to be familiar with the location and shape of a certain set of common functions. For eample, from our previous knowledge about linear equations, ou can determine the location of the linear function f () a b. You know that the graph of this function is a line whose slope is a and whose intercept is (0, b). The eight graphs in Figure.5 represent some of the most commonl used functions in algebra. You should be familiar with the characteristics of the graphs of these common functions. This will help ou predict and analze the graphs of more complicated functions that are derived from appling one or more transformations to these simple functions. There are other important basic functions with which ou should be familiar for eample, eponential, logarithmic and eponential functions but we will encounter these in later chapters. f() f() Hint: When analzing the graph of a function, it is often convenient to epress a function in the form f (). As we have done throughout this chapter, we often refer to a function such as f () b the equation. Figure.5 Common functions f() f() c a) Constant function b) Identit function c) Absolute value function d) Squaring function f() f() f() f() e) Square root function f) Cubing function g) Inverse function h) Inverse square function 5
23 Functions and equations Downloaded from Hint: The word inverse can have different meanings in mathematics depending on the contet. In Section. of this chapter, inverse is used to describe operations or functions that undo each other. However, inverse is sometimes used to denote the multiplicative inverse (or reciprocal) of a number or function. This is how it is used in the names for the functions shown in (g) and (h) of Figure.5. The function in g) is sometimes called the reciprocal function. We will see that man functions have graphs that are a transformation (translation, reflection or stretch), or a combination of transformations, of one of these common functions. Vertical and horizontal translations Use our GDC to graph each of the following three functions: f (), g () and h(). How do the graphs of g and h compare with the graph of f that is one of the common functions displaed in Figure.5? The graphs of g and h both appear to have the same shape it s onl the location, or position, that has changed compared to f. Although the curves (parabolas) appear to be getting closer together, their vertical separation at ever value of is constant. Plot Y= X Y= X + Y= X  Y= Y5= Y6= Y7= Plot Plot (, ) (, 9) (, 9) (, 7) (, 7) (, ) (, ) (, ) (, ) (, ) (, ) (, ) Figure.6 Figure.7 As Figures.6 and.7 clearl show, ou can obtain the graph of g () b translating (shifting) the graph of f () up three units, and ou can obtain the graph of h () b translating the graph of f () down two units. Vertical translations of a function Given k 0, then: I. The graph of f () k is obtained b translating up k units the graph of f (). II. The graph of f () k is obtained b translating down k units the graph of f (). Change function g to g () ( ) and change function h to h () ( ). Graph these two functions along with the parent function 5
24 f () on our GDC. This time we observe that the functions g and h can be obtained b a horizontal translation of f. ( ) Plot Y= X Plot Plot Y=(X + ) Y=(X  ) Y= Y5= Y6= Y7= ( 5, ) (, ) (0, 9) (, 9) Note that a different graphing stle is assigned to each equation on the GDC. Figure.8 (, 9) (5, 9) (, ) (, ) ( ) Figure.9 As Figures.8 and.9 clearl show, ou can obtain the graph of g () ( ) b translating the graph of f () three units to the left, and ou can obtain the graph of h () ( ) b translating the graph of f () two units to the right. Horizontal translations of a function Given k 0, then: I. The graph of f ( h) is obtained b translating the graph of f () h units to the right. II. The graph of f ( h) is obtained b translating the graph of f () h units to the left. 55
25 Functions and equations Downloaded from Hint: A common error is caused b confusion about the direction of a horizontal translation since f () is translated left if a positive number is added inside the argument of the function e.g. g () ( ) is obtained b translating f () three units left. You are in the habit of associating positive with movement to the right (as on the ais) instead of left. Whereas f () is translated up if a positive number is added outs ide the function e.g. g () is obtained b translating f () three units up. This agrees with the convention that a positive number is associated with an upward movement (as on the ais). An alternative (and more consistent) approach to vertical and horizontal translations is to think of what number is being added directl to the  or coordinate. For eample, the equation for the graph obtained b translating the graph of three units up is, which can also be written as. In this form, negative three is added to the coordinate (vertical coordinate), which causes a vertical translation in the upward (or positive) direction. Likewise, the equation for the graph obtained b translating the graph of two units to the right is ( ). Negative two is added to the coordinate (horizontal coordinate), which causes a horizontal translation to the right (or positive direction). There is consistenc between vertical and horizontal translations. Assuming that movement up or to the right is considered positive, and that movement down or to the left is negative, then the direction for either tpe of translation is opposite to the sign ( ) of the number being added to the vertical () or horizontal () coordinate. In fact, what is actuall being translated is the ais or the ais. For eample, the graph of can also be obtained b not changing the graph of but instead translating the ais three units down which creates eactl the same effect as translating the graph of three units up. Note that in Eample 8, if the transformations had been performed in reverse order that is, the vertical translation followed b the horizontal translation it would produce the same final graph (in part (b)) with the same equation. In other words, when appling both a vertical and horizontal translation on a function it does not make an difference which order the are applied (i.e. the are commutative). However, as we will see further on in the chapter, it can make a difference how other sequences of transformations are applied. In general, transformations are not commutative. Eample 8 The diagrams show how the graph of is transformed to the graph of f () in three steps. For each diagram, a) and b), give the equation of the curve. f() a) b) To obtain graph a), the graph of is translated three units to the right. To produce the equation of the translated graph, is added inside the argument of the function. Therefore, the equation of the curve graphed in a) is. To obtain graph b), the graph of is translated up one unit. To produce the equation of the translated graph, is added outside the function. Therefore, the equation of the curve graphed in b) is (or ). ( 5, 0) (, 0) (, ) 56 Eample 9 Write the equation of the absolute value function whose graph is shown on the left. The graph shown is eactl the same shape as the graph of the equation but in a different position. Given that the verte is (, ), it is clear that this graph can be obtained b translating two units left
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