# Solutions to Exercises, Section 5.1

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1 Instructor s Solutions Manual, Section 5.1 Exercise 1 Solutions to Exercises, Section Find all numbers t such that ( 1 3,t) is a point on the unit circle. For ( 1 3,t)to be a point on the unit circle means that the sum of the squares of the coordinates equals 1. In other words, ( 1 ) t 2 = 1. This simplifies to the equation t 2 = 8 9, which implies that t = 8 3 or 8 t = 3. Because 8 = 4 2 = 4 2 = 2 2, we can rewrite this as t = or t =

2 Student Solution Manual, Section 5.1 Exercise 3 3. Find all numbers t such that (t, 2 5 ) is a point on the unit circle. For (t, 2 5 ) to be a point on the unit circle means that the sum of the squares of the coordinates equals 1. In other words, t 2 + ( 2 5 ) 2 = 1. This simplifies to the equation t 2 = 21 25, which implies that t = 21 5 or t = 21 5.

3 Instructor s Solutions Manual, Section 5.1 Exercise 5 5. Find the points where the line through the origin with slope 3 intersects the unit circle. The line through the origin with slope 3 is characterized by the equation y = 3x. Substituting this value for y into the equation for the unit circle (x 2 + y 2 = 1) gives x 2 + (3x) 2 = 1, which simplifies to the equation 10x 2 10 = 1. Thus x = 10 or x = Using each of these values of x along with the equation y = 3x gives the points ( 10 10, 3 10 ) ( and 10, 3 10 ) 10 as the points of intersection of the line y = 3x and the unit circle.

4 Student Solution Manual, Section 5.1 Exercise 7 7. Suppose an ant walks counterclockwise on the unit circle from the point (1, 0) to the endpoint of the radius that forms an angle of 70 with the positive horizontal axis. How far has the ant walked? We need to find the length of the circular arc on the unit circle corresponding to a 70 angle. This length equals 70π 7π 180, which equals 18.

5 Instructor s Solutions Manual, Section 5.1 Exercise 9 9. What angle corresponds to a circular arc on the unit circle with length π 5? Let θ be such that the angle of θ degrees corresponds to an arc on the unit circle with length π θπ 5. Thus 180 = π 5. Solving this equation for θ, wegetθ = 36. Thus the angle in question is 36.

6 Student Solution Manual, Section 5.1 Exercise What angle corresponds to a circular arc on the unit circle with length 5 2? Let θ be such that the angle of θ degrees corresponds to an arc on the unit circle with length 5 θπ 2. Thus 180 = 5 2. Solving this equation for θ, we get θ = π. Thus the angle in question is π, which is approximately equal to

7 Instructor s Solutions Manual, Section 5.1 Exercise Find the lengths of both circular arcs on the unit circle connecting the points (1, 0) and ( 2 2, 2 ) 2. The radius of the unit circle ending at the point ( 2 2, 2 ) 2 makes an angle of 45 with the positive horizontal axis. One of the circular arcs connecting (1, 0) and ( 2 2, 2 ) 2 is shown below as the thickened circular arc; the other circular arc connecting (1, 0) and ( 2 2, 2 ) 2 is the unthickened part of the unit circle below. The length of the thickened arc below is 45π 180, which equals π 4. The entire unit circle has length 2π. Thus the length of the other circular arc below is 2π π 4, which equals 7π The thickened circular arc has length π 4. The other circular arc has length 7π 4.

8 Student Solution Manual, Section 5.1 Exercise 15 For each of the angles in Exercises 15 20, find the endpoint of the radius of the unit circle that makes the given angle with the positive horizontal axis The radius making a 120 angle with the positive horizontal axis is shown below. The angle from this radius to the negative horizontal axis equals , which equals 60 as shown in the figure below. Drop a perpendicular line segment from the endpoint of the radius to the horizontal axis, forming a right triangle as shown below. We already know that one angle of this right triangle is 60 ; thus the other angle must be 30, as labeled below: The side of the right triangle opposite the 30 angle has length 1 2 ; the side of the right triangle opposite the 60 angle has length 3 2. Looking at the figure above, we see that the first coordinate of the endpoint of the

9 Instructor s Solutions Manual, Section 5.1 Exercise 15 radius is the negative of the length of the side opposite the 30 angle, and the second coordinate of the endpoint of the radius is the length of the side opposite the 60 angle. Thus the endpoint of the radius is ( 1 2, 3 ) 2.

10 Student Solution Manual, Section 5.1 Exercise The radius making a 30 angle with the positive horizontal axis is shown below. Draw a perpendicular line segment from the endpoint of the radius to the horizontal axis, forming a right triangle as shown below. We already know that one angle of this right triangle is 30 ; thus the other angle must be 60, as labeled below. The side of the right triangle opposite the 30 angle has length 1 2 ; the side of the right triangle opposite the 60 3 angle has length 2. Looking at the figure below, we see that the first coordinate of the endpoint of the radius is the length of the side opposite the 60 angle, and the second coordinate of the endpoint of the radius is the negative of the length of the side opposite the 30 angle. Thus the endpoint of the radius is ( 3 2, 1 )

11 Instructor s Solutions Manual, Section 5.1 Exercise The radius making a 390 angle with the positive horizontal axis is obtained by starting at the horizontal axis, making one complete counterclockwise rotation, and then continuing for another 30. The resulting radius is shown below. Drop a perpendicular line segment from the endpoint of the radius to the horizontal axis, forming a right triangle as shown below. We already know that one angle of this right triangle is 30 ; thus the other angle must be 60, as labeled below. The side of the right triangle opposite the 30 angle has length 1 2 ; the side opposite the 60 3 angle has length 2. Looking at the figure below, we see that the first coordinate of the endpoint of the radius is the length of the side opposite the 60 angle, and the second coordinate of the endpoint of the radius is the length of the side opposite the 30 angle. Thus the endpoint of the radius is ( 3 2, 1 )

12 Student Solution Manual, Section 5.1 Exercise 21 For Exercises 21 26, find the angle the radius of the unit circle ending at the given point makes with the positive horizontal axis. Among the infinitely many possible correct s, choose the one with the smallest absolute value. ( , 3 ) 2 Draw the radius whose endpoint is ( 1 2, 3 ) 2. Drop a perpendicular line segment from the endpoint of the radius to the horizontal axis, forming a right triangle. The hypotenuse of this right triangle is a radius of the unit circle and thus has length 1. The horizontal side has length 1 2 and the vertical side of this triangle has length 3 2 because the endpoint of the radius is ( 1 2, 3 ) Thus we have a triangle, with the 30 angle opposite the horizontal side of length 1 2, as labeled above. Because = 120,

13 Instructor s Solutions Manual, Section 5.1 Exercise 21 the radius makes a 120 angle with the positive horizontal axis, as shown above. In addition to making a 120 angle with the positive horizontal axis, this radius also makes with the positive horizontal axis angles of 480, 840, and so on. This radius also makes with the positive horizontal axis angles of 240, 600, and so on. But of all the possible choices for this angle, the one with the smallest absolute value is 120.

14 Student Solution Manual, Section 5.1 Exercise ( 2 2, 2 ) 2 Draw the radius whose endpoint is ( 2 2, 2 ) 2. Draw a perpendicular line segment from the endpoint of the radius to the horizontal axis, forming a right triangle. The hypotenuse of this right triangle is a radius of the unit circle and thus has length 1. The horizontal side has 2 length 2 and the vertical side of this triangle also has length 2 2 because the endpoint of the radius is ( 2 2, 2 ) Thus we have here an isosceles right triangle, with two angles of 45 as labeled above. In addition to making a 45 angle with the positive horizontal axis, this radius also makes with the positive horizontal axis angles of 315, 675, and so on. This radius also makes with the positive horizontal axis angles of 405, 765, and so on. But of all the possible choices for this angle, the one with the smallest absolute value is 45.

15 Instructor s Solutions Manual, Section 5.1 Exercise ( 2 2, 2 ) 2 Draw the radius whose endpoint is ( 2 2, 2 ) 2. Draw a perpendicular line segment from the endpoint of the radius to the horizontal axis, forming a right triangle. The hypotenuse of this right triangle is a radius of the unit circle and thus has length 1. The horizontal side has 2 length 2 and the vertical side of this triangle also has length 2 2 because the endpoint of the radius is ( 2 2, 2 ) Thus we have here an isosceles right triangle, with two angles of 45 as labeled above. Because the radius makes a 45 angle with the negative horizontal axis, it makes a 135 angle with the positive horizontal axis, as shown below (because 135 = ). In addition to making a 135 angle with the positive horizontal axis, this radius also makes with the positive horizontal axis angles of 225, 585, and so on. This radius also makes with the positive horizontal axis

16 Student Solution Manual, Section 5.1 Exercise 25 angles of 495, 855, and so on. But of all the possible choices for this angle, the one with the smallest absolute value is 135.

17 Instructor s Solutions Manual, Section 5.1 Exercise Find the lengths of both circular arcs on the unit circle connecting the point ( 1 2, 3 ) 2 and the point that makes an angle of 130 with the positive horizontal axis. The radius of the unit circle ending at the point ( 1 2, 3 ) 2 makes an angle of 60 with the positive horizontal axis. One of the circular arcs connecting ( 1 2, 3 ) 2 and the point that makes an angle of 130 with the positive horizontal axis is shown below as the thickened circular arc; the other circular arc connecting these two points is the unthickened part of the unit circle below. The thickened arc below corresponds to an angle of 70 (because 70 = ). Thus the length of the thickened arc below is 70π 180, which equals 7π 18. The entire unit circle has length 2π. Thus the length of the other circular arc below is 2π 7π 29π 18, which equals The thickened circular arc has length 7π The other circular arc has length 29π

18 Student Solution Manual, Section 5.1 Exercise Find the lengths of both circular arcs on the unit circle connecting the point ( 2 2, 2 ) 2 and the point that makes an angle of 125 with the positive horizontal axis. The radius of the unit circle ending at ( 2 2, 2 ) 2 makes an angle of 225 with the positive horizontal axis (because 225 = ). One of the circular arcs connecting ( 2 2, 2 ) 2 and the point that makes an angle of 125 with the positive horizontal axis is shown below as the thickened circular arc; the other circular arc connecting these two points is the unthickened part of the unit circle below. The thickened arc below corresponds to an angle of 100 (because 100 = ). Thus the length of the thickened arc below is 100π 180, which equals 5π 9. The entire unit circle has length 2π. Thus the length of the other circular arc below is 2π 5π 13π 9, which equals The thickened circular arc has length 5π 9. The other circular arc has length 13π 9.

19 Instructor s Solutions Manual, Section 5.1 Exercise What is the slope of the radius of the unit circle that has a 30 angle with the positive horizontal axis? The radius of the unit circle that has a 30 angle with the positive horizontal axis has its initial point at (0, 0) and its endpoint at ( 3 2, 1 ) 2. Thus the slope of this radius is which equals 1 3, which equals ,

20 Student Solution Manual, Section 5.2 Exercise 1 Solutions to Exercises, Section 5.2 In Exercises 1 8, convert each angle to radians Start with the equation Divide both sides by 360 to obtain 360 = 2π radians. 1 = π 180 radians. Now multiply both sides by 15, obtaining 15 = 15π 180 radians = π 12 radians.

21 Instructor s Solutions Manual, Section 5.2 Exercise Start with the equation Divide both sides by 360 to obtain 360 = 2π radians. 1 = π 180 radians. Now multiply both sides by 45, obtaining 45 = 45π 180 radians = π 4 radians.

22 Student Solution Manual, Section 5.2 Exercise Start with the equation Divide both sides by 360 to obtain 360 = 2π radians. 1 = π 180 radians. Now multiply both sides by 270, obtaining 270 = 270π 180 radians = 3π 2 radians.

23 Instructor s Solutions Manual, Section 5.2 Exercise Start with the equation Divide both sides by 360 to obtain 360 = 2π radians. 1 = π 180 radians. Now multiply both sides by 1080, obtaining 1080 = 1080π 180 radians = 6π radians.

24 Student Solution Manual, Section 5.2 Exercise 9 In Exercises 9 16, convert each angle to degrees. 9. 4π radians Start with the equation Multiply both sides by 2, obtaining 2π radians = π radians = = 720.

25 Instructor s Solutions Manual, Section 5.2 Exercise π 9 radians Start with the equation 2π radians = 360. Divide both sides by 2 to obtain π radians = 180. Now divide both sides by 9, obtaining π radians = = 20. 9

26 Student Solution Manual, Section 5.2 Exercise radians Start with the equation Divide both sides by 2π to obtain 2π radians = radian = 180. π Now multiply both sides by 3, obtaining 3 radians = π = 540. π

27 Instructor s Solutions Manual, Section 5.2 Exercise π 3 radians Start with the equation 2π radians = 360. Divide both sides by 2 to obtain π radians = 180. Now multiply both sides by 2 3, obtaining 2π 3 radians = = 120.

29 Instructor s Solutions Manual, Section 5.2 Exercise Find the lengths of both circular arcs of the unit circle connecting the point (1, 0) and the endpoint of the radius that makes an angle of 3 radians with the positive horizontal axis. Because 3 is a bit less than π, the radius that makes an angle of 3 radians with the positive horizontal axis lies a bit above the negative horizontal axis, as shown below. The thickened circular arc corresponds to an angle of 3 radians and thus has length 3. The entire unit circle has length 2π. Thus the length of the other circular arc is 2π 3, which is approximately

30 Student Solution Manual, Section 5.2 Exercise Find the lengths of both circular arcs of the unit circle connecting the point ( 2 2, 2 ) 2 and the point whose radius makes an angle of 1 radian with the positive horizontal axis. The radius of the unit circle whose endpoint equals ( 2 2, 2 ) 2 makes an angle of π 4 radians with the positive horizontal axis, as shown with the clockwise arrow below. The radius that makes an angle of 1 radian with the positive horizontal axis is shown with a counterclockwise arrow. 1 Thus the thickened circular arc above corresponds to an angle of 1 + π 4 and thus has length 1 + π 4, which is approximately The entire unit circle has length 2π. Thus the length of the other circular arc below is 2π (1 + π 4 ), which equals 7π 4 1, which is approximately 4.50.

31 Instructor s Solutions Manual, Section 5.2 Exercise For a 16-inch pizza, find the area of a slice with angle 3 4 radians. Pizzas are measured by their diameters; thus this pizza has a radius of 8 inches. Thus the area of the slice is , which equals 24 square inches.

32 Student Solution Manual, Section 5.2 Exercise Suppose a slice of a 12-inch pizza has an area of 20 square inches. What is the angle of this slice? This pizza has a radius of 6 inches. Let θ denote the angle of this slice, measured in radians. Then 20 = 1 2 θ 62. Solving this equation for θ, wegetθ = 10 9 radians.

33 Instructor s Solutions Manual, Section 5.2 Exercise Suppose a slice of pizza with an angle of 5 6 radians has an area of 21 square inches. What is the diameter of this pizza? Let r denote the radius of this pizza. Thus 21 = r 2. Solving this equation for r, we get r = the pizza is approximately 14.2 inches Thus the diameter of

34 Student Solution Manual, Section 5.2 Exercise 29 For each of the angles in Exercises 29 34, find the endpoint of the radius of the unit circle that makes the given angle with the positive horizontal axis π 6 radians For this exercise it may be easier to convert to degrees. Thus we translate 5π 6 radians to 150. The radius making a 150 angle with the positive horizontal axis is shown below. The angle from this radius to the negative horizontal axis equals , which equals 30 as shown in the figure below. Drop a perpendicular line segment from the endpoint of the radius to the horizontal axis, forming a right triangle as shown below. We already know that one angle of this right triangle is 30 ; thus the other angle must be 60, as labeled below. The side of the right triangle opposite the 30 angle has length 1 2 ; the side of the right triangle opposite the 60 angle has length 3 2. Looking at the figure below, we see that the first coordinate of the endpoint of the radius is the negative of the length of the side opposite the 60 angle, and the second coordinate of the endpoint of the radius is the length of the side opposite the 30 angle. Thus the endpoint of the radius is ( 3 2, 1 ) 2.

35 Instructor s Solutions Manual, Section 5.2 Exercise

36 Student Solution Manual, Section 5.2 Exercise π 4 radians For this exercise it may be easier to convert to degrees. Thus we translate π 4 radians to 45. The radius making a 45 angle with the positive horizontal axis is shown below. Draw a perpendicular line segment from the endpoint of the radius to the horizontal axis, forming a right triangle as shown below. We already know that one angle of this right triangle is 45 ; thus the other angle must also be 45, as labeled below. The hypotenuse of this right triangle is a radius of the unit circle and 2 thus has length 1. The other two sides each have length 2. Looking at the figurebelow, we see that the first coordinate of the endpoint of the 2 radius is 2 and the second coordinate of the endpoint of the radius is 2 2. Thus the endpoint of the radius is ( 2 2, 2 )

37 Instructor s Solutions Manual, Section 5.2 Exercise π 2 radians Note that 5π 2 = 2π + π 2. Thus the radius making an angle of 5π 2 radians with the positive horizontal axis is obtained by starting at the horizontal axis, making one complete counterclockwise rotation (which is 2π radians), and then continuing for another π 2 radians. The resulting radius is shown below. Its endpoint is (0, 1). 1

38 Student Solution Manual, Section 5.3 Exercise 1 Solutions to Exercises, Section 5.3 Give exact values for the quantities in Exercises Do not use a calculator for any of these exercises otherwise you will likely get decimal approximations for some s rather than exact answers. More importantly, good understanding will come from working these exercises by hand. 1. (a) cos 3π (b) sin 3π Because 3π = 2π + π, an angle of 3π radians (as measured counterclockwise from the positive horizontal axis) consists of a complete revolution around the circle (2π radians) followed by another π radians (180 ), as shown below. The endpoint of the corresponding radius is ( 1, 0). Thus cos 3π = 1 and sin 3π = 0. 1

39 Instructor s Solutions Manual, Section 5.3 Exercise 3 3. (a) cos 11π 4 (b) sin 11π 4 Because 11π 4 = 2π + π 2 + π 11π 4, an angle of 4 radians (as measured counterclockwise from the positive horizontal axis) consists of a complete revolution around the circle (2π radians) followed by another π 2 radians (90 ), followed by another π 4 radians (45 ), as shown below. Hence the endpoint of the corresponding radius is ( Thus cos 11π 4 = 2 2 and sin 11π 4 = , 2 2 ). 1

40 Student Solution Manual, Section 5.3 Exercise 5 5. (a) cos 2π 3 (b) sin 2π 3 Because 2π 3 = π 2 + π 6, an angle of 2π 3 radians (as measured counterclockwise from the positive horizontal axis) consists of π 2 radians (90 radians) followed by another π 6 radians (30 ), as shown below. The endpoint of the corresponding radius is ( 1 2, 3 ) 2π 2. Thus cos 3 = 1 2 and sin 2π 3 =

41 Instructor s Solutions Manual, Section 5.3 Exercise 7 7. (a) cos 210 (b) sin 210 Because 210 = , an angle of 210 (as measured counterclockwise from the positive horizontal axis) consists of 180 followed by another 30, as shown below. The endpoint of the corresponding radius is ( 3 2, 1 ) 2. Thus cos 210 = 3 2 and sin 210 =

42 Student Solution Manual, Section 5.3 Exercise 9 9. (a) cos (b) sin Because = , an angle of (as measured counterclockwise from the positive horizontal axis) consists of 1000 complete revolutions around the circle followed by another 45. The endpoint of the corresponding radius is ( 2 2, 2 ) 2. Thus cos = 2 2 and sin = 2 2.

43 Instructor s Solutions Manual, Section 5.3 Exercise Find the smallest number θ larger than 4π such that cos θ = 0. Note that 0 = cos π 2 = cos 3π 2 = cos 5π 2 =... and that the only numbers whose cosine equals 0 are of the form (2n+1)π 2, where n is an integer. The smallest number of this form larger than 4π is 9π 2. Thus 9π 2 is the smallest number larger than 4π whose cosine equals 0.

44 Student Solution Manual, Section 5.3 Exercise Find the four smallest positive numbers θ such that cos θ = 0. Think of a radius of the unit circle whose endpoint is (1, 0). If this radius moves counterclockwise, forming an angle of θ with the positive horizontal axis, the first coordinate of its endpoint first becomes 0 when θ equals π 2 (which equals 90 ), then again when θ equals 3π 2 (which equals 270 ), then again when θ equals 5π 2 (which equals ,or 450 ), then again when θ equals 7π 2 (which equals , or 630 ), and so on. Thus the four smallest positive numbers θ such that cos θ = 0 are π 2, 3π 2, 5π 2, and 7π 2.

45 Instructor s Solutions Manual, Section 5.3 Exercise Find the four smallest positive numbers θ such that sin θ = 1. Think of a radius of the unit circle whose endpoint is (1, 0). If this radius moves counterclockwise, forming an angle of θ with the positive horizontal axis, then the second coordinate of its endpoint first becomes 1 when θ equals π 2 (which equals 90 ), then again when θ equals 5π 2 (which equals , or 450 ), then again when θ equals 9π 2 (which equals , or 810 ), then again when θ equals 13π 2 (which equals , or 1170 ), and so on. Thus the four smallest positive numbers θ such that sin θ = 1 are π 2, 5π 2, 9π 13π 2, and 2.

46 Student Solution Manual, Section 5.3 Exercise Find the four smallest positive numbers θ such that cos θ = 1. Think of a radius of the unit circle whose endpoint is (1, 0). If this radius moves counterclockwise, forming an angle of θ with the positive horizontal axis, the first coordinate of its endpoint first becomes 1 when θ equals π (which equals 180 ), then again when θ equals 3π (which equals , or 540 ), then again when θ equals 5π (which equals , or 900 ), then again when θ equals 7π (which equals , or 1260 ), and so on. Thus the four smallest positive numbers θ such that cos θ = 1 are π, 3π, 5π, and 7π.

47 Instructor s Solutions Manual, Section 5.3 Exercise Find the four smallest positive numbers θ such that sin θ = 1 2. Think of a radius of the unit circle whose endpoint is (1, 0). If this radius moves counterclockwise, forming an angle of θ with the positive horizontal axis, the second coordinate of its endpoint first becomes 1 2 when θ equals π 6 (which equals 30 ), then again when θ equals 5π 6 (which equals 150 ), then again when θ equals 13π 6 (which equals ,or 390 ), then again when θ equals 17π 6 (which equals , or 510 ), and so on. Thus the four smallest positive numbers θ such that sin θ = 1 2 are π 6, 5π 6, 13π 17π 6, and 6.

48 Student Solution Manual, Section 5.3 Exercise Suppose 0 <θ< π 2 and cos θ = 2 5. Evaluate sin θ. We know that (cos θ) 2 + (sin θ) 2 = 1. Thus (sin θ) 2 = 1 (cos θ) 2 ( 2 ) 2 = 1 5 = Because 0 <θ< π 2, we know that sin θ>0. Thus taking square roots of both sides of the equation above gives sin θ = 21 5.

49 Instructor s Solutions Manual, Section 5.3 Exercise Suppose π 2 <θ<π and sin θ = 2 9. Evaluate cos θ. We know that (cos θ) 2 + (sin θ) 2 = 1. Thus (cos θ) 2 = 1 (sin θ) 2 ( 2 ) 2 = 1 9 = Because π 2 <θ<π, we know that cos θ<0. Thus taking square roots of both sides of the equation above gives cos θ = 77 9.

50 Student Solution Manual, Section 5.3 Exercise Suppose π 2 <θ<0 and cos θ = 0.1. Evaluate sin θ. We know that (cos θ) 2 + (sin θ) 2 = 1. Thus (sin θ) 2 = 1 (cos θ) 2 = 1 (0.1) 2 = Because π 2 <θ<0, we know that sin θ<0. Thus taking square roots of both sides of the equation above gives sin θ =

51 Instructor s Solutions Manual, Section 5.3 Exercise Find the smallest number x such that sin(e x ) = 0. Note that e x is an increasing function. Because e x is positive for every real number x, and because π is the smallest positive number whose sine equals 0, we want to choose x so that e x = π. Thus x = ln π.

52 Student Solution Manual, Section 5.3 Exercise Find the smallest positive number x such that sin(x 2 + x + 4) = 0. Note that x 2 + x + 4 is an increasing function on the interval [0, ). If x is positive, then x 2 + x + 4 > 4. Because 4 is larger than π but less than 2π, the smallest number bigger than 4 whose sine equals 0is2π. Thus we want to choose x so that x 2 + x + 4 = 2π. In other words, we need to solve the equation x 2 + x + (4 2π) = 0. Using the quadratic formula, we see that the s to this equation are x = 1 ± 8π A calculator shows that choosing the plus sign in the equation above gives x and choosing the minus sign gives x We seek only positive values of x, and thus we choose the plus sign in the equation above, getting x

53 Instructor s Solutions Manual, Section 5.4 Exercise 1 Solutions to Exercises, Section Find the four smallest positive numbers θ such that tan θ = 1. Think of a radius of the unit circle whose endpoint is (1, 0). If this radius moves counterclockwise, forming an angle of θ with the positive horizontal axis, then the first and second coordinates of its endpoint first become equal (which is equivalent to having tan θ = 1) when θ equals π 4 (which equals 45 ), then again when θ equals 5π 4 (which equals 225 ), then again when θ equals 9π 4 (which equals , or 405 ), then again when θ equals 13π 4 (which equals , or 585 ), and so on. Thus the four smallest positive numbers θ such that tan θ = 1 are π 4, 5π 4, 9π 13π 4, and 4.

54 Student Solution Manual, Section 5.4 Exercise 3 3. Suppose 0 <θ< π 2 and cos θ = 1 5. Evaluate: (a) sin θ exercise: (b) tan θ The figure below gives a sketch of the angle involved in this Θ 1 The angle between 0 and π 2 equals 1 5. whose cosine (a) We know that (cos θ) 2 + (sin θ) 2 = 1. Thus ( 1 5 ) 2 + (sin θ) 2 = 1. Solving this equation for (sin θ) 2 gives (sin θ) 2 = The sketch above shows that sin θ>0. Thus taking square roots of both sides of the equation above gives

55 Instructor s Solutions Manual, Section 5.4 Exercise 3 sin θ = = = (b) tan θ = sin θ cos θ = = 2 6.

56 Student Solution Manual, Section 5.4 Exercise 5 5. Suppose π 2 <θ<π and sin θ = 2 3. Evaluate: (a) cos θ exercise: (b) tan θ The figure below gives a sketch of the angle involved in this Θ 1 The angle between π 2 equals 2 3. and π whose sine (a) We know that (cos θ) 2 + (sin θ) 2 = 1. Thus (cos θ) 2 + ( 2 3 ) 2 = 1. Solving this equation for (cos θ) 2 gives (cos θ) 2 = 5 9. The sketch above shows that cos θ<0. Thus taking square roots of both sides of the equation above gives

57 Instructor s Solutions Manual, Section 5.4 Exercise 5 cos θ = 5 3. (b) tan θ = sin θ cos θ = = 2 5 =

58 Student Solution Manual, Section 5.4 Exercise 7 7. Suppose π 2 <θ<0 and cos θ = 4 5. Evaluate: (a) sin θ exercise: (b) tan θ The figure below gives a sketch of the angle involved in this Θ 1 The angle between π 2 cosine equals 4 5. and 0 whose (a) We know that (cos θ) 2 + (sin θ) 2 = 1. Thus ( 4 5 ) 2 + (sin θ) 2 = 1. Solving this equation for (sin θ) 2 gives (sin θ) 2 = The sketch above shows that sin θ<0. Thus taking square roots of both sides of the equation above gives

59 Instructor s Solutions Manual, Section 5.4 Exercise 7 sin θ = 3 5. (b) tan θ = sin θ cos θ = = 3 4.

60 Student Solution Manual, Section 5.4 Exercise 9 9. Suppose 0 <θ< π 2 and tan θ = 1 4. Evaluate: (a) cos θ exercise: (b) sin θ The figure below gives a sketch of the angle involved in this Θ 1 The angle between 0 and π 2 whose tangent equals 1 4. (a) Rewrite the equation tan θ = 1 sin θ 4 in the form cos θ = 1 4. Multiplying both sides of this equation by cos θ, weget sin θ = 1 4 cos θ. Substitute this expression for sin θ into the equation (cos θ) 2 +(sin θ) 2 = 1, getting (cos θ) (cos θ)2 = 1, which is equivalent to

61 Instructor s Solutions Manual, Section 5.4 Exercise 9 (cos θ) 2 = The sketch above shows that cos θ>0. Thus taking square roots of both sides of the equation above gives cos θ = 4 17 = (b) We have already noted that sin θ = 1 4 cos θ. Thus sin θ =

62 Student Solution Manual, Section 5.4 Exercise Suppose π 2 (a) cos θ <θ<0 and tan θ = 3. Evaluate: (b) sin θ exercise: The figure below gives a sketch of the angle involved in this Θ 1 The angle between π 2 tangent equals 3. and 0 whose (a) Rewrite the equation tan θ = 3 in the form sin θ cos θ = 3. Multiplying both sides of this equation by cos θ, weget sin θ = 3 cos θ. Substitute this expression for sin θ into the equation (cos θ) 2 +(sin θ) 2 = 1, getting (cos θ) 2 + 9(cos θ) 2 = 1, which is equivalent to

63 Instructor s Solutions Manual, Section 5.4 Exercise 11 (cos θ) 2 = The sketch above shows that cos θ>0. Thus taking square roots of both sides of the equation above gives cos θ = 1 10 = (b) We have already noted that sin θ = 3 cos θ. Thus sin θ =

64 Student Solution Manual, Section 5.4 Exercise 13 Given that cos 15 = and sin 22.5 = 2 2, 2 in Exercises find exact expressions for the indicated quantities. [These values for cos 15 and sin 22.5 will be derived in Examples 4 and 5 in Section 6.3.] 13. sin 15 We know that (cos 15 ) 2 + (sin 15 ) 2 = 1. Thus (sin 15 ) 2 = 1 (cos 15 ) 2 ( = 1 2 = = Because sin 15 > 0, taking square roots of both sides of the equation above gives 2 3 sin 15 =. 2 ) 2

65 Instructor s Solutions Manual, Section 5.4 Exercise tan 15 tan 15 sin 15 = cos = = = = 2 3

66 Student Solution Manual, Section 5.4 Exercise cot 15 cot 15 1 = tan 15 1 = = = = 2 + 3

67 Instructor s Solutions Manual, Section 5.4 Exercise csc 15 csc 15 = = 1 sin = = =

68 Student Solution Manual, Section 5.4 Exercise sec 15 sec 15 = = 1 cos = = = 2 2 3

69 Instructor s Solutions Manual, Section 5.4 Exercise 23 Suppose u and ν are in the interval (0, π 2 ), with tan u = 2 and tan ν = 3. In Exercises 23 32, find exact expressions for the indicated quantities. 23. cot u cot u = 1 tan u = 1 2

70 Student Solution Manual, Section 5.4 Exercise cos u We know that 2 = tan u = sin u cos u. To find cos u, make the substitution sin u = 1 (cos u) 2 in the equation above (this substitution is valid because we know that 0 <u< π 2 and thus sin u>0), getting 1 (cos u) 2 2 =. cos u Now square both sides of the equation above, then multiply both sides by (cos u) 2 and rearrange to get the equation 5(cos u) 2 = 1. Because 0 <u< π 2, we see that cos u>0. Thus taking square roots of both sides of the equation above gives cos u = 1 5, which can be rewritten as cos u = 5 5.

71 Instructor s Solutions Manual, Section 5.4 Exercise sin u sin u = 1 (cos u) 2 = 4 = 5 = = 2 5 5

72 Student Solution Manual, Section 5.4 Exercise csc u csc u = 1 sin u 5 = 2

73 Instructor s Solutions Manual, Section 5.4 Exercise sec u sec u = 1 cos u = 5

74 Student Solution Manual, Section 5.4 Exercise Find the smallest number x such that tan e x = 0. Note that e x is an increasing function. Because e x is positive for every real number x, and because π is the smallest positive number whose tangent equals 0, we want to choose x so that e x = π. Thus x = ln π

75 Instructor s Solutions Manual, Section 5.5 Exercise 1 Solutions to Exercises, Section 5.5 Use the right triangle below for Exercises This triangle is not drawn to scale corresponding to the data in the exercises. c Ν b u a 1. Suppose a = 2 and b = 7. Evaluate c. The Pythagorean Theorem implies that c 2 = Thus c = = 53.

76 Student Solution Manual, Section 5.5 Exercise 3 3. Suppose a = 2 and b = 7. Evaluate cos u. cos u = adjacent side hypotenuse = a c = 2 =

77 Instructor s Solutions Manual, Section 5.5 Exercise 5 5. Suppose a = 2 and b = 7. Evaluate sin u. sin u = opposite side hypotenuse = b c = 7 =

78 Student Solution Manual, Section 5.5 Exercise 7 7. Suppose a = 2 and b = 7. Evaluate tan u. tan u = opposite side adjacent side = b a = 7 2

79 Instructor s Solutions Manual, Section 5.5 Exercise 9 9. Suppose a = 2 and b = 7. Evaluate cos ν. cos ν = adjacent side hypotenuse = b c = 7 =

80 Student Solution Manual, Section 5.5 Exercise Suppose a = 2 and b = 7. Evaluate sin ν. sin ν = opposite side hypotenuse = a c = 2 =

81 Instructor s Solutions Manual, Section 5.5 Exercise Suppose a = 2 and b = 7. Evaluate tan ν. tan ν = opposite side adjacent side = a b = 2 7

82 Student Solution Manual, Section 5.5 Exercise Suppose b = 2 and c = 7. Evaluate a. The Pythagorean Theorem implies that a = 7 2. Thus a = = 45 = 9 5 = 9 5 = 3 5.

83 Instructor s Solutions Manual, Section 5.5 Exercise Suppose b = 2 and c = 7. Evaluate cos u. adjacent side cos u = hypotenuse = a c = 3 5 7

84 Student Solution Manual, Section 5.5 Exercise Suppose b = 2 and c = 7. Evaluate sin u. sin u = opposite side hypotenuse = b c = 2 7

85 Instructor s Solutions Manual, Section 5.5 Exercise Suppose b = 2 and c = 7. Evaluate tan u. tan u = opposite side adjacent side = b a = =

86 Student Solution Manual, Section 5.5 Exercise Suppose b = 2 and c = 7. Evaluate cos ν. cos ν = adjacent side hypotenuse = b c = 2 7

87 Instructor s Solutions Manual, Section 5.5 Exercise Suppose b = 2 and c = 7. Evaluate sin ν. opposite side sin ν = hypotenuse = a c = 3 5 7

88 Student Solution Manual, Section 5.5 Exercise Suppose b = 2 and c = 7. Evaluate tan ν. opposite side tan ν = adjacent side = a b = 3 5 2

89 Instructor s Solutions Manual, Section 5.5 Exercise Suppose a = 5 and u = 17. Evaluate b. We have tan 17 = opposite side adjacent side = b 5. Solving for b, wegetb = 5 tan

90 Student Solution Manual, Section 5.5 Exercise Suppose a = 5 and u = 17. Evaluate c. We have cos 17 = adjacent side hypotenuse = 5 c. Solving for c, we get c = 5 cos

91 Instructor s Solutions Manual, Section 5.5 Exercise Suppose u = 17. Evaluate cos ν. Because ν = 90 u, we have ν = 73. Thus cos ν = cos

92 Student Solution Manual, Section 5.5 Exercise Suppose u = 17. Evaluate sin ν. sin ν = sin

93 Instructor s Solutions Manual, Section 5.5 Exercise Suppose u = 17. Evaluate tan ν. tan ν = tan

94 Student Solution Manual, Section 5.5 Exercise Suppose c = 8 and u = 1 radian. Evaluate a. We have cos 1 = adjacent side hypotenuse = a 8. Solving for a, we get a = 8 cos When using a calculator to do the approximation above, be sure that your calculator is set to operate in radian mode.

95 Instructor s Solutions Manual, Section 5.5 Exercise Suppose c = 8 and u = 1 radian. Evaluate b. We have sin 1 = opposite side hypotenuse = b 8. Solving for b, we get b = 8 sin

96 Student Solution Manual, Section 5.5 Exercise Suppose u = 1 radian. Evaluate cos ν. Because ν = π 2 u, we have ν = π 2 1. Thus cos ν = cos( π 2 1)

97 Instructor s Solutions Manual, Section 5.5 Exercise Suppose u = 1 radian. Evaluate sin ν. sin ν = sin( π 2 1) 0.540

98 Student Solution Manual, Section 5.5 Exercise Suppose u = 1 radian. Evaluate tan ν. tan ν = tan( π 2 1) 0.642

99 Instructor s Solutions Manual, Section 5.5 Exercise Suppose c = 4 and cos u = 1 5. Evaluate a. We have Solving this equation for a, we get 1 adjacent side = cos u = 5 hypotenuse = a 4. a = 4 5.

100 Student Solution Manual, Section 5.5 Exercise Suppose c = 4 and cos u = 1 5. Evaluate b. The Pythagorean Theorem implies that ( 4 5 ) 2 + b 2 = 4 2. Thus b = = = =

101 Instructor s Solutions Manual, Section 5.5 Exercise Suppose cos u = 1 5. Evaluate sin u. sin u = 1 (cos u) 2 = = 25 = 2 6 5

102 Student Solution Manual, Section 5.5 Exercise Suppose cos u = 1 5. Evaluate tan u. tan u = sin u cos u = = 2 6

103 Instructor s Solutions Manual, Section 5.5 Exercise Suppose cos u = 1 5. Evaluate cos ν. cos ν = b c = sin u = 2 6 5

104 Student Solution Manual, Section 5.5 Exercise Suppose cos u = 1 5. Evaluate sin ν. sin ν = a c = cos u = 1 5

105 Instructor s Solutions Manual, Section 5.5 Exercise Suppose cos u = 1 5. Evaluate tan ν. tan ν = sin ν cos ν = = = 12

106 Student Solution Manual, Section 5.5 Exercise Suppose b = 4 and sin ν = 1 3. Evaluate a. We have 1 opposite side = sin ν = 3 hypotenuse = a c. Thus c = 3a. By the Pythagorean Theorem, we also have c 2 = a Substituting 3a for c in this equation gives 9a 2 = a Solving the equation above for a shows that a = 2.

107 Instructor s Solutions Manual, Section 5.5 Exercise Suppose b = 4 and sin ν = 1 3. Evaluate c. Thus We have 1 3 = sin ν = a c. c = 3a = 3 2.

108 Student Solution Manual, Section 5.5 Exercise Suppose sin ν = 1 3. Evaluate cos u. cos u = a c = sin ν = 1 3

109 Instructor s Solutions Manual, Section 5.5 Exercise Suppose sin ν = 1 3. Evaluate sin u. sin u = ( 1 ) 2 1 (cos u) 2 = 1 3 = 8 9 = 2 2 3

110 Student Solution Manual, Section 5.5 Exercise Suppose sin ν = 1 3. Evaluate tan u. tan u = sin u cos u = = 2 2

111 Instructor s Solutions Manual, Section 5.5 Exercise Suppose sin ν = 1 3. Evaluate cos ν. cos ν = ( 1 ) 2 1 (sin ν) 2 = 1 3 = 8 9 = 2 2 3

112 Student Solution Manual, Section 5.5 Exercise Suppose sin ν = 1 3. Evaluate tan ν. tan ν = sin ν cos ν = = = 4

113 Instructor s Solutions Manual, Section 5.5 Exercise Suppose a 25-foot ladder is leaning against a wall, making a 63 angle with the ground (as measured from a perpendicular line from the base of the ladder to the wall). How high up the wall is the end of the ladder? In the sketch here, the vertical line represents the wall and the hypotenuse represents the ladder. As labeled here, the ladder touches the wall at height b; thus we need to evaluate b. 25 b We have sin 63 = b 25. Solving this equation for b, weget 63 b = 25 sin Thus the ladder touches the wall at a height of approximately feet. Because = 3.36, this is approximately 22 feet, 3 inches.

114 Student Solution Manual, Section 5.5 Exercise Suppose you need to find the height of a tall building. Standing 20 meters away from the base of the building, you aim a laser pointer at the closest part of the top of the building. You measure that the laser pointer is 4 tilted from pointing straight up. The laser pointer is held 2 meters above the ground. How tall is the building? In the sketch here, the rightmost vertical line represents the building and the hypotenuse represents the path of the laser beam. Because the laser pointer is 4 tilted from pointing straight up, the b angle formed by the laser beam and a line parallel to the ground is 86, as indicated in the figure (which is not drawn to scale). The side of the right triangle opposite the 86 angle has been labeled b. Thus the height of the building is b + 2. We have tan 86 = b 20. Solving this equation for b, weget b = 20 tan Adding 2 to this result, we see that the height of the building is approximately 288 meters.

115 Instructor s Solutions Manual, Section 5.6 Exercise 1 Solutions to Exercises, Section For θ = 7, evaluate each of the following: (a) cos 2 θ (b) cos(θ 2 ) [Exercises 1 and 2 emphasize that cos 2 θ does not equal cos(θ 2 ).] (a) Using a calculator working in degrees, we have cos 2 7 = (cos 7 ) 2 ( ) (b) Note that 7 2 = 49. Using a calculator working in degrees, we have cos

116 Student Solution Manual, Section 5.6 Exercise 3 3. For θ = 4 radians, evaluate each of the following: (a) sin 2 θ (b) sin(θ 2 ) [Exercises 3 and 4 emphasize that sin 2 θ does not equal sin(θ 2 ).] (a) Using a calculator working in radians, we have sin 2 4 = (sin 4) 2 ( ) (b) Note that 4 2 = 16. Using a calculator working in radians, we have sin

117 Instructor s Solutions Manual, Section 5.6 Exercise 5 In Exercises 5 38, find exact expressions for the indicated quantities, given that cos π 12 = and sin π 2 8 = [These values for cos π 12 and sin π 8 Section 6.3.] will be derived in Examples 4 and 5 in 5. cos( π 12 ) cos( π 12 ) = cos π 12 =

118 Student Solution Manual, Section 5.6 Exercise 7 7. sin π 12 We know that cos 2 π 12 + sin2 π 12 = 1. Thus Because sin π 12 above gives sin 2 π 12 = 1 cos2 π 12 ( = 1 2 = = > 0, taking square roots of both sides of the equation sin π =. 2 ) 2

119 Instructor s Solutions Manual, Section 5.6 Exercise 9 9. sin( π 12 ) sin( π 12 ) = sin π 12 = 2 3 2

120 Student Solution Manual, Section 5.6 Exercise tan π 12 tan π 12 = sin π 12 cos π = = = = 2 3

121 Instructor s Solutions Manual, Section 5.6 Exercise tan( π 12 ) tan( π 12 ) = tan π 12 = (2 3) = 3 2

122 Student Solution Manual, Section 5.6 Exercise cos 25π 12 Because 25π 12 = π π, we have cos 25π 12 = cos( π π) = cos π =. 2

123 Instructor s Solutions Manual, Section 5.6 Exercise sin 25π 12 Because 25π 12 = π π, we have sin 25π 12 = sin( π π) = sin π =. 2

124 Student Solution Manual, Section 5.6 Exercise tan 25π 12 Because 25π 12 = π π, we have tan 25π 12 = tan( π π) = tan π 12 = 2 3.

125 Instructor s Solutions Manual, Section 5.6 Exercise cos 13π 12 Because 13π 12 = π 12 + π, we have cos 13π 12 = cos( π 12 + π) = cos π =. 2

126 Student Solution Manual, Section 5.6 Exercise sin 13π 12 Because 13π 12 = π 12 + π, we have sin 13π 12 = sin( π 12 + π) = sin π =. 2

127 Instructor s Solutions Manual, Section 5.6 Exercise tan 13π 12 Because 13π 12 = π 12 + π, we have tan 13π 12 = tan( π 12 + π) = tan π 12 = 2 3.

128 Student Solution Manual, Section 5.6 Exercise cos 5π 12 cos 5π 12 = sin( π 2 5π 12 ) = sin π = 2

129 Instructor s Solutions Manual, Section 5.6 Exercise cos( 5π 12 ) cos( 5π 5π ) = cos 12 = 2

130 Student Solution Manual, Section 5.6 Exercise sin 5π 12 sin 5π 12 = cos( π 2 5π 12 ) = cos π = 2

131 Instructor s Solutions Manual, Section 5.6 Exercise sin( 5π 12 ) sin( 5π 12 ) = sin 5π 12 =

132 Student Solution Manual, Section 5.6 Exercise tan 5π 12 tan 5π 12 = 1 tan( π 2 5π 12 ) = 1 tan π 12 1 = = = = 2 + 3

133 Instructor s Solutions Manual, Section 5.6 Exercise tan( 5π 12 ) tan( 5π 5π 12 ) = tan 12 = 2 3

134 Student Solution Manual, Section 5.6 Exercise 39 Suppose u and ν are in the interval ( π 2,π), with tan u = 2 and tan ν = 3. In Exercises 39 66, find exact expressions for the indicated quantities. 39. tan( u) tan( u) = tan u = ( 2) = 2

135 Instructor s Solutions Manual, Section 5.6 Exercise cos u We know that 2 = tan u = sin u cos u. To find cos u, make the substitution sin u = 1 cos 2 u in the equation above (this substitution is valid because π 2 <u<π, which implies that sin u>0), getting 1 cos 2 = 2 u. cos u Now square both sides of the equation above, then multiply both sides by cos 2 u and rearrange to get the equation 5 cos 2 u = 1. Thus cos u = 1 5 (the possibility that cos u equals 1 5 is eliminated because π 2 <u<π, which implies that cos u<0). This can be written as cos u = 5 5.

136 Student Solution Manual, Section 5.6 Exercise cos( u) 5 cos( u) = cos u = 5

137 Instructor s Solutions Manual, Section 5.6 Exercise sin u sin u = 1 cos 2 u = 4 = 5 = = 2 5 5

138 Student Solution Manual, Section 5.6 Exercise sin( u) sin( u) = sin u = 2 5 5

139 Instructor s Solutions Manual, Section 5.6 Exercise cos(u + 4π) 5 cos(u + 4π) = cos u = 5

140 Student Solution Manual, Section 5.6 Exercise sin(u 6π) sin(u 6π) = sin u = 2 5 5

141 Instructor s Solutions Manual, Section 5.6 Exercise tan(u + 8π) tan(u + 8π) = tan u = 2

142 Student Solution Manual, Section 5.6 Exercise cos(u 3π) 5 cos(u 3π) = cos u = 5

143 Instructor s Solutions Manual, Section 5.6 Exercise sin(u + 5π) sin(u + 5π) = sin u = 2 5 5

144 Student Solution Manual, Section 5.6 Exercise tan(u 9π) tan(u 9π) = tan u = 2

145 Instructor s Solutions Manual, Section 5.6 Exercise cos( π 2 u) cos( π 2 u) = sin u = 2 5 5

146 Student Solution Manual, Section 5.6 Exercise sin( π 2 u) sin ( π 2 u) = cos u = 5 5

147 Instructor s Solutions Manual, Section 5.6 Exercise tan( π 2 u) tan( π 2 u) = 1 tan u = 1 2

148 Student Solution Manual, Section 5.7 Exercise 1 Solutions to Exercises, Section Evaluate cos cos π 3 = 1 2 ; thus cos = π 3.

149 Instructor s Solutions Manual, Section 5.7 Exercise 3 3. Evaluate tan 1 ( 1). tan( π 4 ) = 1; thus tan 1 ( 1) = π 4.

150 Student Solution Manual, Section 5.7 Exercise 5 c Ν b u a Use the right triangle above for Exercises This triangle is not drawn to scale corresponding to the data in the exercises. 5. Suppose a = 2 and c = 3. Evaluate u in radians. Because the cosine of an angle in a right triangle equals the length of the adjacent side divided by the length of the hypotenuse, we have cos u = 2 3. Using a calculator working in radians, we then have u = cos radians.

151 Instructor s Solutions Manual, Section 5.7 Exercise 7 7. Suppose a = 2 and c = 5. Evaluate ν in radians. Because the sine of an angle in a right triangle equals the length of the opposite side divided by the length of the hypotenuse, we have sin ν = 2 5. Using a calculator working in radians, we then have ν = sin radians.

152 Student Solution Manual, Section 5.7 Exercise 9 9. Suppose a = 5 and b = 4. Evaluate u in degrees. Because the tangent of an angle in a right triangle equals the length of the opposite side divided by the length of the adjacent side, we have tan u = 4 5. Using a calculator working in degrees, we then have u = tan

153 Instructor s Solutions Manual, Section 5.7 Exercise Suppose a = 5 and b = 7. Evaluate ν in degrees. Because the tangent of an angle in a right triangle equals the length of the opposite side divided by the length of the adjacent side, we have tan ν = 5 7. Using a calculator working in degrees, we then have ν = tan

154 Student Solution Manual, Section 5.7 Exercise Find the smallest positive number t such that 10 cos t = 6. The equation above implies that cos t = log 6. Thus we take t = cos 1 (log 6)

155 Instructor s Solutions Manual, Section 5.7 Exercise Find the smallest positive number t such that e tan t = 15. The equation above implies that tan t = ln 15. Thus we take t = tan 1 (ln 15)

156 Student Solution Manual, Section 5.7 Exercise Find the smallest positive number y such that cos(tan y) = 0.2. The equation above implies that we should choose tan y = cos Thus we should choose y tan

157 Instructor s Solutions Manual, Section 5.7 Exercise Find the smallest positive number x such that sin 2 x 3 sin x + 1 = 0. as Write y = sin x. Then the equation above can be rewritten y 2 3y + 1 = 0. Using the quadratic formula, we find that the s to this equation are y = and y = Thus sin x or sin x However, there is no real number x such that sin x (because sin x is at most 1 for every real number x), and thus we must have sin x Thus x sin

158 Student Solution Manual, Section 5.7 Exercise Find the smallest positive number x such that cos 2 x 0.5 cos x = 0. as Write y = cos x. Then the equation above can be rewritten y 2 0.5y = 0. Using the quadratic formula or factorization, we find that the s to this equation are y = 0.2 and y = 0.3. Thus cos x = 0.2 orcosx = 0.3, which suggests that we choose x = cos or x = cos Because arccosine is a decreasing function, cos is smaller than cos Because we want to find the smallest positive value of x satisfying the original equation, we choose x = cos

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