# LINEAR FUNCTIONS OF 2 VARIABLES

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1 CHAPTER 4: LINEAR FUNCTIONS OF 2 VARIABLES 4.1 RATES OF CHANGES IN DIFFERENT DIRECTIONS From Precalculus, we know that is a linear function if the rate of change of the function is constant. I.e., for ever unit that we move in the direction, the rise in the direction is constant. Eample Consider the function given b the following table: For each movement of 1 in the direction, the output f() from the function increases b 3. Hence, the function is linear with slope (rate of change) equal to three. Eample Consider the function given b the following table: From 0 to 1, the output f() increases b 2. From 1 to 2, the output f() increases b 6. Hence the rate of change of the function is not constant and the function is not linear. RATES OF CHANGE IN THE X AND Y DIRECTIONS In the case of functions of two variables we also have a notion of rate of change. However, there are now two variables, and, and so we will consider two rates of change: a rate of change associated with movement in the direction and a rate of change associated with movement in the direction. 89

2 Eample Consider the situation we saw in the previous chapter where both parents in a household work and the father earns \$5.00 an hour while the mother earns \$10.00 an hour. As before we shall be using: number of hours that the mother works in a week number of hours that the father works in a week weekl salar for the famil For each increase of one unit in, the value of the function increases b 10. That is, for each hour that the mother works in a week, the weekl famil salar increases b 10 dollars. Hence the rate of change in the direction (we refer to this as the slope in the direction or m ) is 10. Similarl, for each increase of one unit in, the value of the function increases b 5. That is, for each increase of one in the number of hours the father works the famil salar increases b 5 dollars. Hence the rate of change in the direction (we refer to this as the slope in the direction or m ) is DEFINITION OF A LINEAR FUNCTION OF TWO VARIABLES DEFINITION A function of two variables is said to be linear if it has a constant rate of change in the direction and a constant rate of change in the direction. We will normall epress this idea as m and m are constant. 4.3 RECOGNIZING A LINEAR FUNCTION OF TWO VARIABLES SURFACES If a linear function is represented with a surface, the surface will have a constant slope in the direction and the direction. Such a surface is represented below. 90

3 On the above surface, in the directions the cross sections are a series of lines with slope a and in the direction, the cross sections are a series of lines with slope b. This is consistent with constant slopes in the and in the directions. If we visualize a surface with constant slopes in the and directions, the surface that represents a linear function in 3 dimensions will alwas be a plane. TABLES Assume that the function f is represented b the following table: = 0 = 1 = 2 = 3 = = = = Geometricall, we can see the information contained in the table b first placing a point for each (, ) in the table on the plane of our 3-space 91

4 Then we can raise each point to its appropriate z value (height) in 3 dimensions. If this is a plane, an two points in the direction should give us the same slope. For eample, if we go from (0, 0, 0) to (1, 0, 10), we can obtain one slope in the direction. This slope is equal rise 10 to m 10. If we go from (1, 1, 15) to (3, 1, 35), this will provide another slope in run 1 92

5 rise 20 the direction. This slope is equal to m 10. If we obtain the slope in this manner run 2 for an two points that are oriented in the direction, we will find that the all result in a slope of 10. Hence, the slope in the direction for the data points in this table are all equal to 10. If this is a plane, an two points in the direction should also give us the same slope. For eample, if we go from (0, 0, 0) to (0, 1, 5), we can obtain one slope in the direction. This rise 5 slope is equal to m 5. If we go from (1, 1, 15) to (1, 3, 25), this will provide run 1 rise 30 another slope in the direction. This slope is equal to m 5. If we obtain the run 3 slope in this manner for an two points that are oriented in the direction, we will find that the all result in a slope of 5. Hence, the slope in the direction for the data points in this table are all equal to 5. As the slopes in the direction that are associated with these points and the slopes in the direction associated with these points are both constant, this plane is consistent with the definition of a linear function. CONTOUR DIAGRAMS Assume that the function f is represented b the following contour diagram: If this is a plane, an two points in the direction should give us the same slope. For eample, if we go from (2, 0, 2) to (2, 0, 3), we can obtain one slope in the direction. This slope is equal to rise 1 m 1. If we go from (1, 2, 3) to (2, 2, 4), this will provide another slope in the run 1 rise 1 direction. This slope is equal to m 1. If we obtain the slope in this manner for an run 1 two points that are oriented in the direction, we will find that the all result in a slope of 1. Hence, the slopes in the direction for the data points in this table are all equal to 1. 93

6 If this is a plane, an two points in the direction should also give us the same slope. For eample, if we go from (2, 0, 2) to (2, 1, 3), we can obtain one slope in the direction. This rise 1 slope is equal to m 1. If we go from (1, 1, 2) to (1, 3, 4), this will provide another run 1 rise 2 slope in the direction. This slope is equal to m 1. If we obtain the slope in this run 2 manner for an two points that are oriented in the direction, we will find that the all result in a slope of 1. Hence, the slope in the direction for the data points in this table are all equal to 1. As the slopes in the direction that are associated with these points and the slopes in the direction associated with these points are both constant, this contour is consistent with the definition of a linear function. Geometricall, we can see the information contained in the contour b moving each contour on the plane to its appropriate height in 3-space With these contours in 3-space, we can see that the contour diagram is consistent with the following plane. 94

7 FORMULAS Given a formula, the ke to determining whether slopes are constant in the and directions lies in the fact that when we move in the direction, is constant and when we move in the direction is constant. For eample, the above trajector starts at (1, 4) and moves in the direction through the points (2, 4), (3, 4) and (4, 4). While increases with a trajector in the direction, the value of remains constant at = 4. 95

8 In the above case, the trajector starts at (2, 2) and moves in the direction through the points (2, 3), (2, 4) and (2, 5). While increases with a trajector in the direction, the value of remains constant at = 2. Eample Eercise 4.3.1: Given the function represented with the formula z = f(,) = +, determine whether the formula represents a linear function. Solution: Assume that we start at an point (a, b) on the plane. If we move in the direction then remains constant in = b and the cross section of the curve will be z = + b where b is constant. This cross section is a line with slope equal to 1. Hence the slope in the direction is equal to 1 for ever point (a, b). For eample, when we start at the point (0, 0) and move in the direction, our trajector will be associated with the cross section = 0 where z = + 0. If we move in the direction then remains constant in = a and the cross section of the curve will be z = a + where a is constant. This cross section is a line with slope equal to 1. Hence the slope in the direction is equal to 1 for ever point (a, b). For eample, when we start at the point (0, 0) and move in the direction, our trajector will be associated with the cross section = 0 where z =

9 Hence, the function represented b the formula z = f(,) = + has constant slopes in both the direction and the direction and does represent a linear function. If we note that the point (0, 0, 0) satisfies the function and place the appropriate cross sections in the and directions, we can see that the associated plane will have the following form: Eample Eercise 4.3.2: Given the function represented with the formula z = f(,) = + 2, determine whether the formula represents a linear function. Solution: Assume that we start at an point (a, b) on the plane. If we move in the direction then remains constant in = b and the cross section of the curve will be z = + 3b 2 +1 where b is constant. This cross section is a line with slope equal to 2. Hence the slope in the direction is equal to 1 for ever point (a, b). 97

10 Eample: = 0 z = f (, 0) = =. This is z =. z If we move in the direction then remains constant in = a and the cross section of the curve will be z = + a 2 where a is constant. This cross section is a parabola. As a parabola is not a curve with a constant slope, the slope in the direction is not constant. If = 0 z = = 2. This is the equation of a parabola that open toward them z > 0 with verte (0, 0) on the z plane. If = 1 z = This is the equation of a parabola that open toward them z > 1 with verte (0, 1) on the z plane. If = 2 z = This is the equation of a parabola that open toward them z > 2 with verte (0, 2) on the z plane. 98

11 Hence, the function represented b the formula z = f(,) = + 2 has a constant slope in the direction but does not have a constant slope in the direction and does not represent a linear function. 4.4 MOVEMENT ON PLANES AND THE ASSOCIATED CHANGE IN HEIGHT Eample Eercise 4.4.1: Given a plane with m = 1 and m = 2, find the difference in height between (0, 0, f (0, 0)) and (2, 3, f (2, 3)). Solution: 1. Identif the right triangle whose rise and run are associated with this slope. 99

12 2. Divide the trajector into movement in the direction and movement in the direction. 3. Find the change in height associated with each component of the trajector: Rise in the direction: 100

13 2, m 1 z 2 Rise in the direction: 101

14 3, m 2 z 6 4. Find the overall change in height: Taking both components of the change in height, the total difference in height is z z 8. Generalization If m is known and m is known then the difference in height between an two points on a plane can be obtained with the following steps. 1. Identif the right triangle whose rise and run are associated with this slope. 102

15 2. Divide the trajector into movement in the direction and movement in the direction. 3. Find the change in height associated with each component of the trajector. 103

16 Rise in the direction: The rise in the direction z m. In a similar manner we can find that the rise in the direction z m. 104

17 Conclusion: Taking both components of the change in height, the total difference in height is z z z m m. 105

18 4.5 PLANES AND SLOPES IN VARIOUS DIRECTIONS Eample Eercise 4.5.1: Given a plane with m = 1 and m = 2, find the slope in the direction of the vector <2, 3>. We will refer to this as m <2,3>. Solution: 2. Identif the right triangle whose rise and run are associated with this slope. 3. Obtain the rise for 2,

19 o 2, m 1 z 2 o 3, m 2 z 6 o z z z Obtain the run for, 2, 3. Using Pthagoras and the above right triangle, run

20 5. Obtain the slope. m rise run

21 Generalization If m and m are known then the slope in an direction can be obtained with the following steps. 1. Obtain the rise using,, m, and m. o Rise in the direction: z m o Rise in the direction: z m o Total rise: z z z m m 2. Obtain the run for and. run

22 3. Obtain the slope: m rise run m m FORMULAS FOR A PLANE POINT SLOPE FORMULA Recall from Precalculus that there are various formulas to represent the same line. One of them is where m is the slope of the line and is a point on the line. This is usuall written as and is called the point-slope formula. This formula can be generalized to get a formula for a plane as shown below. Suppose that we are given a point on a plane and that the slopes in the direction and in the direction, and respectivel, are known. We want a formula that will relate the coordinates of an arbitrar point on the plane to the rest of the known information. To find this, recall from the previous section that the change in height from the given point to the generic point can be obtained b first moving in the direction and then moving in the direction and adding the corresponding changes in height. That is, 110

23 But we saw in section 4.4 that and so that the change in height ma be written as in the following bo. Point-Slopes Formula for a Plane An equation for a plane that contains the point 0, 0, z 0 and has slopes m and directions respectivel is m in the and z z m m Eample Suppose that in the following table, we are given some values of a linear function. The problem is to find a formula for f,. \ We know that since the function is linear the graph must be a plane. Looking at the table we see that the slopes in the and directions are m 2 and m 3. Further, the table gives us nine points we can choose from, so that we ma use the Points-Slopes Formula. Take for eample 1,1,9. Then appling the formula we get: which simplifies to z z Of course we would have gotten the same formula had we used an other point on the table. 111

24 THE SLOPES-INTERCEPT FORMULA FOR A PLANE Another formula for a line seen in Chapter 1 was the slope-intercept formula: m b where m is the slope and b is the -intercept of the line. This is a useful formula to represent a line when the slope and the -intercept of the line are known. This formula can be generalized to get a formula for a plane. Note that in the Point-Slopes Formula, if the known point happens to be the z intercept 0,0,c then plugging into the formula produces: 0 0 z c m m Simplifing ields the Slopes-Intercept Formula for a plane shown in the bo below. Slopes-Intercept Formula for a Plane The equation of a plane that crosses the z ais at c and that has slopes directions respectivel is z m m c. m and m in the and Note the slopes-intercept formula for a plane has the form: z a b c. You ma think of this as a generalization of the formula a b for lines, where now ou have two slopes and an intercept instead of one slope and an intercept as it was with lines. Eample: From a Formula to a Geometric Plane Consider the plane z The cross-section corresponding to 0 is z with a slope of 3. The cross-section corresponding to This is a line in the direction is z This is a line in the direction with a slope of 2. Also, observe that the z intercept must be 4. This ma be observed b taking both 0 and 0 z to get To summarize we have obtained the following three data: The slope in the direction: m 2. The slope in the direction: m 3. The point 0,0, 4 satisfies the equation. 112

25 We can now geometricall represent this information b placing the point 0,0, 4 in 3-space and as we know that for ever step in the direction, our height increases b 2 and for ever step in the direction our height increases b 3, we can place lines in the direction and in the direction consistent with this information. The result is illustrated in Figure With this information, we can place the unique plane which will satisf m 2, m 3 and have z intercept at 0,0, 4 (see Figure 4.6.2). Figure Figure Eample Eercise 4.6.1: Find a formula for the function in following table: \ Solution: We can easil verif the following three facts: 113

26 For ever step we take in the direction (that is, 1 and 0 ), our height increases b 2 irrespective of where we begin. This is the slope in the direction: m 2. For ever step we take in the direction (that is, 0 and 1), our height increases b 3 irrespective of where we begin. This is the slope in the direction: m 3. The point 0,0, 4 satisfies the equation. Hence, using the Slopes-Intercept Formula, the equation is z Eample Eercise 4.6.2: From a Geometric Plane to the Slopes-Intercept Formula Consider the following plane: Solution: Observe that: The point 0,0,10 lies on the plane hence the z intercept is 10. Upon moving 2 steps in the direction we've risen 2 units indicating a rise of 1 for each step in the direction or m

27 Upon moving 2 steps in the direction we've risen 8 units indicating a rise of 2 for each step in the direction or m 4. Using the Slopes-Intercept Formula we get that the equation of the plane is z THE POINT-NORMAL FORMULA OF A PLANE In Figure below, we have a point in 3-space,, z and a vector n a, b, c. There is onl one plane that will pass through the point,, z and for which n a, b, c is the perpendicular (also called normal). This is illustrated in Figure Figure Figure Hence one point on a plane and a vector perpendicular to the plane is sufficient to determine a unique plane in three-dimensional space. Our goal is to find the formula for this plane. This formula can be derived through the following steps: Step 1: As ( 0, 0, z 0) lies on the plane, if (,, z ) is an point on the plane other than ( 0, 0, z 0) then, as Figure illustrates, the displacement vector from ( 0, 0, z 0) to (,, z ), 0, 0, z z0, is parallel to the plane, that is, it can be placed on the plane. 115

28 Step 2: If 0, 0, z z0 lies on the plane then it is perpendicular to the normal vector n a, b, c (see Figure 4.6.6). Figure Figure Step 3: As 0, 0, z z0 is perpendicular to abc,, we can conclude that,, z z a, b, c 0 or a b c z z Hence we have: The Point-Normal Formula of a Plane The plane that passes through,, z with normal vector n a, b, c has equation: a b c z z It is worth noting that the above formula can be rewritten as a b cz d where d a b cz. From here it is not hard to show that if we have the formula for a plane epressed as a b cz d then the vector abc,, is perpendicular to the plane. Eample Eercise 4.6.4: Find the equation of the plane that passes through 1,2, 3 and that is normal to the vector 2, 5,4. 116

29 Using the Point-Normal Formula results in: z get 2 5 4z and so simplifing we Eample Eercise 4.6.5: Find a vector that is perpendicular to the plane z Rewriting the equation of the plane in the form a b cz d we get 2 3 z 4. From here we can read off the normal vector 2,3,1. EXERCISE PROBLEMS: 1. For each of the following planes, find i. The slope in the direction ii. The slope in the direction iii. The z intercept iv. A vector perpendicular to the plane v. The slope in the direction NE vi. The slope in the direction <2,3> A. z = B. + 4 z = 3 C z = 5 D. 3 = 2 5z + 3 E. 2 3 = 7z + 3 F. 3 = 2 4z For each of the following data associated with a plane, find i. The slope in the direction ii. The slope in the direction iii. The slope in the direction NE iv. The slope in the direction <3,4> v. The formula for a plane consistent with these data and with z intercept 4 vi. The formula for a plane consistent with these data that passes through (3,2,5). A. The slope towards the east is 3 and the slope towards the north is 2 B. If we move 3 meters east, our altitude rises 6 meters and if we move 2 meters north our altitude rises 10 meters. C. A normal vector to the plane is <1,2,3>. D. A normal vector to the plane points directl upwards. E. The slope towards the west is -3 and the slope towards the north is 4 117

30 F. If we move 3 meters north, our altitude falls 6 meters and if we move 2 meters west, our altitude rises 10 meters. 3. The following diagram represents a plane. The values d and d are displacements in the direction and the direction respectivel. The distances, d1, d2, and d3 are vertical line segments. The lines l1, l2, and l3 are lines on the plane. a. If d = 2, d=3, d1 = 6 and d2 = 4, find the slopes of l1, l2 and l3. b. If d = 4, d=2, d1 = 16 and d2 = 10, find the slopes of l1, l2 and l3. c. If d = 3, d=3, d1 = 6 and d3 = 24, find the slopes of l1, l2 and l3. d. If d = 2, d=3, d2 = 6 and d3 = 24, find the slopes of l1, l2 and l3. e. If d = 2, d=3, the slope in the direction l1 is 3 and the slope in the direction l2 is 5, find the distances d1, d2 and d3 and the slope in the direction l3. f. If d = 4, d=2, the slope in the direction l1 is 2 and the slope in the direction l2 is 3, find the distances d1, d2 and d3 and the slope in the direction l3. g. If d = 4, d=3, the slope in the direction l1 is 5 and the slope in the direction l3 is 7, find the distances d1, d2 and d3 and the slope in the direction l2. 118

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