INVERSE TRIGONOMETRIC FUNCTIONS

Transcription

1 Mathematics, in general, is fundamentally the science of self-evident things. FELIX KLEIN. Introduction In Chapter, we have studied that the inverse of a function f, denoted by f, eists if f is one-one and onto. There are many functions which are not one-one, onto or both and hence we can not talk of their inverses. In Class XI, we studied that trigonometric functions are not one-one and onto over their natural domains and ranges and hence their inverses do not eist. In this chapter, we shall study about the restrictions on domains and ranges of trigonometric functions which ensure the eistence of their inverses and observe their behaviour through graphical representations. Besides, some elementary properties will also be discussed. The inverse trigonometric functions play an import Arya Bhatta ( A. D.) role in calculus for they serve to define many integrals. The concepts of inverse trigonometric functions is also used in science and engineering.. Basic Concepts In Class XI, we have studied trigonometric functions, which are defined as follows: sine function, i.e., sine : R [, ] cosine function, i.e., cos : R [, ] gent function, i.e., : R { : = (n + ), n Z} R cogent function, i.e., cot : R { : = n, n Z} R Chapter INVERSE TRIGONOMETRIC FUNCTIONS secant function, i.e., sec : R { : = (n + ), n Z} R (, ) cosecant function, i.e., cosec : R { : = n, n Z} R (, )

2 4 MATHEMATICS We have also learnt in Chapter that if f : X Y such that f () = y is one-one and onto, then we can define a unique function g : Y X such that g(y) =, where X and y = f (), y Y. Here, the domain of g = range of f and the range of g = domain of f. The function g is called the inverse of f and is denoted by f. Further, g is also one-one and onto and inverse of g is f. Thus, g = (f ) = f. We also have (f o f ) () = f (f ()) = f (y) = and (f o f ) (y) = f (f (y)) = f () = y Since the domain of sine function is the set of all real numbers and range is the closed interval [, ]. If we restrict its domain to,, then it becomes one-one and onto with range [, ]. Actually, sine function restricted to any of the intervals,,,,, etc., is one-one and its range is [, ]. We can, therefore, define the inverse of sine function in each of these intervals. We denote the inverse of sine function by sin (arc sine function). Thus, sin is a function whose domain is [, ] and range could be any of the intervals,,, or,, and so on. Corresponding to each such interval, we get a branch of the function sin. The branch with range, is called the principal value branch, whereas other intervals as range give different branches of sin. When we refer to the function sin, we take it as the function whose domain is [, ] and range is,. We write sin : [, ], From the definition of the inverse functions, it follows that sin (sin ) = if and sin (sin ) = if sin y =.. In other words, if y = sin, then Remarks (i) We know from Chapter, that if y = f () is an invertible function, then = f (y). Thus, the graph of sin function can be obtained from the graph of original function by interchanging and y aes, i.e., if (a, b) is a point on the graph of sine function, then (b, a) becomes the corresponding point on the graph of inverse

3 INVERSE TRIGONOMETRIC FUNCTIONS 5 (ii) of sine function. Thus, the graph of the function y = sin can be obtained from the graph of y = sin by interchanging and y aes. The graphs of y = sin and y = sin are as given in Fig. (i), (ii), (iii). The dark portion of the graph of y = sin represent the principal value branch. It can be shown that the graph of an inverse function can be obtained from the corresponding graph of original function as a mirror image (i.e., reflection) along the line y =. This can be visualised by looking the graphs of y = sin and y = sin as given in the same aes (Fig. (iii)). Fig. (i) Fig. (ii) Fig. (iii) Like sine function, the cosine function is a function whose domain is the set of all real numbers and range is the set [, ]. If we restrict the domain of cosine function to [0, ], then it becomes one-one and onto with range [, ]. Actually, cosine function

4 6 MATHEMATICS restricted to any of the intervals [, 0], [0,], [, ] etc., is bijective with range as [, ]. We can, therefore, define the inverse of cosine function in each of these intervals. We denote the inverse of the cosine function by cos (arc cosine function). Thus, cos is a function whose domain is [, ] and range could be any of the intervals [, 0], [0, ], [, ] etc. Corresponding to each such interval, we get a branch of the function cos. The branch with range [0, ] is called the principal value branch of the function cos. We write cos : [, ] [0, ]. The graph of the function given by y = cos can be drawn in the same way as discussed about the graph of y = sin. The graphs of y = cos and y = cos are given in Fig. (i) and (ii). Fig. (i) Fig. (ii) Let us now discuss cosec and sec as follows: Since, cosec =, the domain of the cosec function is the set { : R and sin n, n Z} and the range is the set {y : y R, y or y } i.e., the set R (, ). It means that y = cosec assumes all real values ecept < y < and is not defined for integral multiple of. If we restrict the domain of cosec function to, {0}, then it is one to one and onto with its range as the set R (, ). Actually, cosec function restricted to any of the intervals, { },, {0},, { } etc., is bijective and its range is the set of all real numbers R (, ).

5 INVERSE TRIGONOMETRIC FUNCTIONS 7 Thus cosec can be defined as a function whose domain is R (, ) and range could be any of the intervals, {0},, { },, { } etc. The function corresponding to the range, {0} is called the principal value branch of cosec. We thus have principal branch as cosec : R (, ), {0} The graphs of y = cosec and y = cosec are given in Fig. (i), (ii). Fig. (i) Fig. (ii) Also, since sec = cos, the domain of y = sec is the set R { : = (n + ), n Z} and range is the set R (, ). It means that sec (secant function) assumes all real values ecept < y < and is not defined for odd multiples of. If we restrict the domain of secant function to [0, ] { }, then it is one-one and onto with

6 8 MATHEMATICS its range as the set R (, ). Actually, secant function restricted to any of the intervals [, 0] { }, [0, ], [, ] { } etc., is bijective and its range is R {, }. Thus sec can be defined as a function whose domain is R (, ) and range could be any of the intervals [, 0] { }, [0, ] { }, [, ] { } etc. Corresponding to each of these intervals, we get different branches of the function sec. The branch with range [0, ] { } is called the principal value branch of the function sec. We thus have sec : R (,) [0, ] { } The graphs of the functions y = sec and y = sec - are given in Fig.4 (i), (ii). Fig.4 (i) Fig.4 (ii) Finally, we now discuss and cot We know that the domain of the function (gent function) is the set { : R and (n +), n Z} and the range is R. It means that function is not defined for odd multiples of. If we restrict the domain of gent function to

7 INVERSE TRIGONOMETRIC FUNCTIONS 9,, then it is one-one and onto with its range as R. Actually, gent function restricted to any of the intervals,,,,, etc., is bijective and its range is R. Thus can be defined as a function whose domain is R and range could be any of the intervals,,,,, and so on. These intervals give different branches of the function. The branch with range, is called the principal value branch of the function. We thus have : R, The graphs of the function y = and y = are given in Fig.5 (i), (ii). Fig.5 (i) Fig.5 (ii) We know that domain of the cot function (cogent function) is the set { : R and n, n Z} and range is R. It means that cogent function is not defined for integral multiples of. If we restrict the domain of cogent function to (0, ), then it is bijective with and its range as R. In fact, cogent function restricted to any of the intervals (, 0), (0, ), (, ) etc., is bijective and its range is R. Thus cot can be defined as a function whose domain is the R and range as any of the

8 40 MATHEMATICS intervals (, 0), (0, ), (, ) etc. These intervals give different branches of the function cot. The function with range (0, ) is called the principal value branch of the function cot. We thus have cot : R (0, ) The graphs of y = cot and y = cot are given in Fig.6 (i), (ii). Fig.6 (i) Fig.6 (ii) The following table gives the inverse trigonometric function (principal value branches) along with their domains and ranges. sin : [, ], cos : [, ] [0, ] cosec : R (,), {0} sec : R (, ) [0, ] { } : R, cot : R (0, )

9 INVERSE TRIGONOMETRIC FUNCTIONS 4 Note. sin should not be confused with (sin ). In fact (sin ) = sin and similarly for other trigonometric functions.. Whenever no branch of an inverse trigonometric functions is mentioned, we mean the principal value branch of that function.. The value of an inverse trigonometric functions which lies in the range of principal branch is called the principal value of that inverse trigonometric functions. We now consider some eamples: Eample Find the principal value of sin. Solution Let sin = y. Then, sin y =. We know that the range of the principal value branch of sin is, and sin 4 =. Therefore, principal value of sin is 4 Eample Find the principal value of cot Solution Let cot = y. Then, cot y = = cot = cot = cot We know that the range of principal value branch of cot is (0, ) and cot =. Hence, principal value of cot is Find the principal values of the following:. sin. cos 4. ( ) 5. cos EXERCISE.. cosec () 6. ( )

10 4 MATHEMATICS 7. sec 0. cosec ( ) Find the values of the following:. () + cos. If sin = y, then (A) 0 y (C) 0 < y < 4. sec ( ) (A) 8. cot ( ) 9. cos + sin is equal to (B) (B) (D) (C). cos + sin y < y < (D). Properties of Inverse Trigonometric Functions In this section, we shall prove some import properties of inverse trigonometric functions. It may be mentioned here that these results are valid within the principal value branches of the corresponding inverse trigonometric functions and wherever they are defined. Some results may not be valid for all values of the domains of inverse trigonometric functions. In fact, they will be valid only for some values of for which inverse trigonometric functions are defined. We will not go into the details of these values of in the domain as this discussion goes beyond the scope of this tet book. Let us recall that if y = sin, then = sin y and if = sin y, then y = sin. This is equivalent to sin (sin ) =, [, ] and sin (sin ) =,, Same is true for other five inverse trigonometric functions as well. We now prove some properties of inverse trigonometric functions.. (i) sin = cosec, or (ii) cos = sec, or

11 INVERSE TRIGONOMETRIC FUNCTIONS 4 (iii) = cot, > 0 To prove the first result, we put cosec = y, i.e., = cosec y Therefore = sin y Hence sin = y or sin = cosec Similarly, we can prove the other parts.. (i) sin ( ) = sin, [, ] (ii) ( ) =, R (iii) cosec ( ) = cosec, Let sin ( ) = y, i.e., = sin y so that = sin y, i.e., = sin ( y). Hence sin = y = sin ( ) Therefore sin ( ) = sin Similarly, we can prove the other parts.. (i) cos ( ) = cos, [, ] (ii) sec ( ) = sec, (iii) cot ( ) = cot, R Let cos ( ) = y i.e., = cos y so that = cos y = cos ( y) Therefore cos = y = cos ( ) Hence cos ( ) = cos Similarly, we can prove the other parts. 4. (i) sin + cos =, [, ] (ii) + cot =, R (iii) cosec + sec =, Let sin = y. Then = sin y = cos Therefore cos = y = y sin

12 44 MATHEMATICS Hence sin + cos = Similarly, we can prove the other parts. + y 5. (i) + y = y, y < (ii) y = + y y, y > (iii) =, < Let = θ and y = φ. Then = θ, y = φ θ+ φ + y Now ( θ+φ ) = = θφ y + y This gives θ + φ = y + y Hence + y = y In the above result, if we replace y by y, we get the second result and by replacing y by, we get the third result. 6. (i) = sin +, (ii) = cos +, 0 (iii) =, < < Let = y, then = y. Now y sin = sin + + y = sin (sin y) = y =

13 INVERSE TRIGONOMETRIC FUNCTIONS 45 Also cos + = cos + (iii) Can be worked out similarly. We now consider some eamples. y y = cos (cos y) = y = Eample Show that (i) sin ( ) = sin, (ii) sin ( ) = cos, Solution (i) Let = sin θ. Then sin = θ. We have sin ( ) = sin ( ) sinθ sin θ = sin (sinθ cosθ) = sin (sinθ) = θ = sin (ii) Take = cos θ, then proceeding as above, we get, sin ( ) Eample 4 Show that + = 4 Solution By property 5 (i), we have = cos L.H.S. = + 5 = = 0 + = = R.H.S. 4 Eample 5 Epress Solution We write cos, sin < < in the simplest form. cos sin cos = cos + sin sin cos sin

14 46 MATHEMATICS = cos + sin cos sin cos sin = cos + sin cos sin + = Alternatively, = + = sin sin cos = = sin cos cos = sin cos 4 4 sin 4 = cot 4 4 = = + 4 = + 4 Eample 6 Write cot, > in the simplest form. Solution Let = sec θ, then = sec θ = θ

15 INVERSE TRIGONOMETRIC FUNCTIONS 47 Therefore, cot = cot (cot θ) = θ = sec, which is the simplest form. Eample 7 Prove that + = Solution Let = θ. Then θ =. We have, < R.H.S. = θ θ θ = = (θ) = θ = = + = + = L.H.S. (Why?) Eample 8 Find the value of cos (sec + cosec ), Solution We have cos (sec + cosec ) = cos = 0 EXERCISE. Prove the following:. sin = sin ( 4 ),,. cos = cos (4 ),,. 7 + = = 7 7 Write the following functions in the simplest form: , 0 6., > cos + cos, < 8. cos sin cos + sin, <

16 48 MATHEMATICS 9. a, < a 0. a a a a, a > 0; Find the values of each of the following:.. cos sin a. cot ( a + cot a) y sin + cos, <, y > 0 and y < + + y 4. If sin sin + cos =, then find the value of If + =, then find the value of + 4 Find the values of each of the epressions in Eercises 6 to sin sin sin + cot 5 7 cos cos is equal to 6 (A) 7 6 (B) 5 6 sin sin ( ) is equal to (A) (B) cot ( ) is equal to (A) (B) 7. 4 (C) (C) 4 (D) (D) 6 (C) 0 (D)

17 Miscellaneous Eamples INVERSE TRIGONOMETRIC FUNCTIONS 49 Eample 9 Find the value of sin (sin ) 5 Solution We know that sin (sin ) =. Therefore, sin (sin ) = 5 5 But, 5, which is the principal branch of sin However sin ( ) = sin( ) = sin and, Therefore sin (sin ) = sin (sin ) = Eample 0 Show that sin sin = cos Solution Let 8 sin = and sin = y 5 7 Therefore sin = and 5 8 sin y = 7 Now 9 4 cos = sin = = (Why?) 5 5 and 64 5 cos y = sin y = = 89 7 We have Therefore cos ( y) = cos cos y + sin sin y = + = y= cos Hence sin sin = cos

18 50 MATHEMATICS Eample Show that 4 6 sin + cos + = 5 6 Solution Let Then 4 6 sin =, cos = y, = z sin =, cos y=, z = 5 6 Therefore 5 cos =, sin y=, = and y= We have + + y 6 ( + y) = = 5 4 = y Hence ( + y) = z i.e., ( + y) = ( z) or ( + y) = ( z) Therefore + y = z or + y = z Since, y and z are positive, + y z (Why?) Hence + y + z = or 4 6 sin + cos + = 5 6 Eample Simplify Solution We have, acos bsin bcos + asin, if a b > acos bsin acos bsin bcos + asin = bcos bcos + asin bcos = a b a + b = a ( ) b = a b

19 INVERSE TRIGONOMETRIC FUNCTIONS 5 Eample Solve + = 4 Solution We have + = 4 or i.e. + = = 4 Therefore 5 = = 6 4 or = 0 i.e., (6 ) ( + ) = 0 which gives = or =. 6 Since = does not satisfy the equation, as the L.H.S. of the equation becomes negative, = is the only solution of the given equation. 6 Find the value of the following: Miscellaneous Eercise on Chapter. cos Prove that sin cos 6. 4 = cos + cos = cos = sin + cos = sin + sin = cos + sin = sin 5 65

20 5 MATHEMATICS Prove that = cos, [0, ] + + sin + sin cot = + sin sin, 0, 4. + = cos + + 4, [Hint: Put = cos θ] sin = sin Solve the following equations:. (cos ) = ( cosec ) sin ( ), < is equal to (A) (B) (C) =,( > 0) + + (D) + 6. sin ( ) sin =, then is equal to (A) 0, (B), (C) 0 (D) 7. y y + y is equal to (A) (B) (C) 4 (D) 4

21 INVERSE TRIGONOMETRIC FUNCTIONS 5 Summary The domains and ranges (principal value branches) of inverse trigonometric functions are given in the following table: Functions Domain Range (Principal Value Branches) y = sin [, ], y = cos [, ] [0, ] y = cosec R (,), {0} y = sec R (, ) [0, ] { } y = R, y = cot R (0, ) sin should not be confused with (sin ). In fact (sin ) = sin and similarly for other trigonometric functions. The value of an inverse trigonometric functions which lies in its principal value branch is called the principal value of that inverse trigonometric functions. For suitable values of domain, we have y = sin = sin y = sin y y = sin sin (sin ) = sin (sin ) = sin = cosec cos ( ) = cos cos = sec cot ( ) = cot = cot sec ( ) = sec

22 54 MATHEMATICS sin ( ) = sin + cot = sin + cos = ( ) = cosec ( ) = cosec cosec + sec = + y + y = y = y y = + y = sin + = cos + Historical Note The study of trigonometry was first started in India. The ancient Indian Mathematicians, Aryabhatta (476A.D.), Brahmagupta (598 A.D.), Bhaskara I (600 A.D.) and Bhaskara II (4 A.D.) got import results of trigonometry. All this knowledge went from India to Arabia and then from there to Europe. The Greeks had also started the study of trigonometry but their approach was so clumsy that when the Indian approach became known, it was immediately adopted throughout the world. In India, the predecessor of the modern trigonometric functions, known as the sine of an angle, and the introduction of the sine function represents one of the main contribution of the siddhantas (Sanskrit astronomical works) to mathematics. Bhaskara I (about 600 A.D.) gave formulae to find the values of sine functions for angles more than 90. A siteenth century Malayalam work Yuktibhasa contains a proof for the epansion of sin (A + B). Eact epression for sines or cosines of 8, 6, 54, 7, etc., were given by Bhaskara II. The symbols sin, cos, etc., for arc sin, arc cos, etc., were suggested by the astronomer Sir John F.W. Hersehel (8) The name of Thales (about 600 B.C.) is invariably associated with height and disce problems. He is credited with the determination of the height of a great pyramid in Egypt by measuring shadows of the pyramid and an auiliary staff (or gnomon) of known

23 INVERSE TRIGONOMETRIC FUNCTIONS 55 height, and comparing the ratios: H S h = = (sun s altitude) s Thales is also said to have calculated the disce of a ship at sea through the proportionality of sides of similar triangles. Problems on height and disce using the similarity property are also found in ancient Indian works.