Applications of Trigonometry

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1 chapter 6 Tides on a Florida beach follow a periodic pattern modeled by trigonometric functions. Applications of Trigonometry This chapter focuses on applications of the trigonometry that was introduced in the last chapter. The chapter begins by showing how trigonometry can be used to compute areas of various regions. Then we will see how trigonometry enables us to compute all the angles and the lengths of all the sides of a triangle given only some of this information. The double-angle and half-angle formulas for the trigonometric functions will allow us to compute exact expressions for quantities such as cos 5 and sin 8. The addition and subtraction formulas for the trigonometric functions will help us discover new identities. Transformations of trigonometric functions are used to model periodic events. Redoing function transformations in the context of trigonometric functions will also help us review the key concepts of function transformations from Chapter. This chapter concludes an optional section giving an introduction to polar coordinates, which are based on trigonometry, and another optional section on vectors and the complex plane. 56

2 section 6. Using Trigonometry to Compute Area Using Trigonometry to Compute Area By the end of this section you should section objectives be able to compute the area of a triangle given the lengths of two sides and the angle between them; understand the ambiguous angle problem that sometimes arises when trying to find the angle between two sides of a triangle; understand the formula for the area of a parallelogram; be able to compute the area of a regular polygon. The Area of a Triangle via Trigonometry Suppose we know the lengths of two sides of a triangle and the angle between those two sides. How can we find the area of the triangle? The example below shows how a knowledge of trigonometry helps solve this problem. Find the area of a triangle that has sides of length and 7 and an angle of 9 between those two sides. example We will consider the side of length 7 to be the base of the triangle. Let h denote the corresponding height of the triangle, as shown here. Looking at the figure above, we see that sin 9 = h. Solving for h, we have h = sin 9. Thus the triangle has area 7h = 7 sin 9 = sin h 7 To find a formula for the area of a triangle given the lengths of two sides of a triangle and the angle between those two sides, we repeat the process used in the example above. We know that the area of a triangle is one-half the base times the height. Thus we will begin by finding a formula for the height of a triangle in terms of the lengths of two sides and the angle between them. Consider a triangle with sides of length a and b and an angle θ between those two sides. We will consider b to be the base of the triangle. Let h denote the corresponding height of this triangle. We want to write the height h in terms of the known measurements of the triangle, which are a, b, and θ. Looking at the figure here, we see that sin θ = h a. Solving for h, we have h = a sin θ. Θ a h b A triangle with base b and height h. The area of the triangle above is bh. Substituting a sin θ for h shows that the area of the triangle equals ab sin θ. Thus we have arrived at our desired formula giving the area of a triangle in terms of the lengths of two sides and the angle between those sides.

3 58 chapter 6 Applications of Trigonometry Area of a triangle A triangle with sides of length a and b and with angle θ between those two sides has area ab sin θ. a b This right triangle has area ab. Whenever we encounter a new formula we should check that it agrees with previously known formulas in cases where both formulas apply. The formula above allows us to compute the area of a triangle whenever we know the lengths of two sides and the angle between those sides. We already knew how to do this when the angle in question is a right angle. Specifically, we already knew that the right triangle shown here has area ab (which is half the area of a rectangle with sides of length a and b). To apply our new formula to the right triangle above, we take θ = π. Because sin π =, the expression ab sin θ becomes ab. In other words, our new formula for the area of a triangle gives the same result as our previous formula for the area of a right triangle. Thus the two formulas are consistent (if they had been inconsistent, then we would know that one of them was incorrect). Ambiguous Angles Suppose a triangle has sides of lengths a and b, an angle θ between those sides, and area R. Given any three of a, b, θ, and R, we can use the equation R = ab sin θ to solve for the other quantity. This process is mostly straightforward the exercises at the end of this section provide some practice in this procedure. However, a subtlety arises when we know the lengths a, b and the area R and we need to find the angle θ. Solving the equation above for sin θ, we get sin θ = R ab. Thus θ is an angle whose sine equals R ab, and it would seem that we finish R by taking θ = sin ab. Sometimes this is correct, but not always. Let s look at an example to see what can happen. example Suppose a triangle with area 6 has sides of lengths and 8. Find the angle between those two sides. Solving for sin θ as above, we have sin θ = R ab = 6 8 =. Now sin equals π 6 radians, which equals 0. Thus it appears that our triangle should look like this:

4 section 6. Using Trigonometry to Compute Area This triangle has area 6. However, the sine of 50 also equals. Thus the following triangle with sides of lengths and 8 also has area 6: 50 8 This triangle also has area 6. If the only information available is that the triangle has area 6 and sides of length and 8, then there is no way to decide which of the two possibilities above truly represents the triangle. Do not mistakenly think that because sin is defined to equal π 6 radians (which equals 0 ), the preferred in the example above is to choose θ = 0. We defined the arcsine of a number to be in the interval [ π, π ] because some choice needed to be made in order to obtain a well-defined inverse for the sine. However, remember that given a number t in [, ], there are angles other than sin t whose sine equals t (although there is only one such angle in the interval [ π, π ]). In the example above, we had 0 and 50 as two angles whose sine equals. More generally, given any number t in [, ] and an angle θ such that sin θ = t, we also have sin(π θ) = t. This follows from the identity sin(π θ) = sin θ, which can be derived as follows: sin(π θ) = ( sin(θ π) ) = ( sin θ) = sin θ, where the first identity above follows from our identity for the sine of the negative of an angle (Section 5.6) and the second identity follows from our formula for the sine of θ + nπ, with n = (also Section 5.6). When working in degrees instead of radians, the result in the paragraph above should be restated to say that the angles θ and (80 θ) have the same sine. Returning to the example above, note that in addition to 0 and 50, there are other angles whose sine equals. For example, 0 and 90 are two such angles. But a triangle cannot have a negative angle, and a triangle cannot have an angle larger than 80. Thus neither 0 nor 90 is a viable possibility for the angle θ in the triangle in question. The two radii shown here have endpoints with the same second coordinate; thus the corresponding angles have the same sine.

5 60 chapter 6 Applications of Trigonometry The Area of a Parallelogram via Trigonometry a h Θ b A parallelogram with base b and height h. The procedure for finding the area of a parallelogram, given an angle of the parallelogram and the lengths of the two adjacent sides, is the same as the procedure followed for a triangle. Consider a parallelogram with sides of length a and b and an angle θ between those two sides, as shown here. We will consider b to be the base of the parallelogram, and we let h denote the height of the parallelogram. We want to write the height h in terms of what we assume are the known measurements of the parallelogram, which are a, b, and θ. Looking at the figure above, we see that sin θ = h a. Solving for h, we have h = a sin θ. The area of the parallelogram above is bh. Substituting a sin θ for h shows that the area equals ab sin θ. Thus we have the following formula: Area of a parallelogram A parallelogram with adjacent sides of length a and b and with angle θ between those two sides has area ab sin θ. Suppose a parallelogram has adjacent sides of lengths a and b, an angle θ between those sides, and area R. Given any three of a, b, θ, and R, we can use the equation R = ab sin θ to solve for the other quantity. As with the case of a triangle, if we know the lengths a and b and the area R, then there can be two possible choices for θ. For a parallelogram, both choices can be correct, as illustrated in the example below. example In a parallelogram that has area 0 and pairs of sides with lengths 5 and 0, as shown here, find the angle between the sides of lengths 5 and 0. Solving the area formula above for sin θ, we have sin θ = R ab = = 5. Θ Θ 5 A calculator shows that sin radians, which is approximately 5.. An angle of π sin 5, which is approximately 6.9, also has a sine equal to 5. To determine whether θ 5. or whether θ 6.9, we need to look at the figure here. As you can see, two angles have been labeled θ both angles are between sides of length 5 and 0, reflecting the ambiguity in the statement of the problem. Although the ambiguity makes this a poorly stated problem, our formula has found both possible answers!

6 section 6. Using Trigonometry to Compute Area 6 Specifically, if what was meant was the acute angle θ above (the leftmost angle labeled θ), then θ 5. ; if what was meant was the obtuse angle θ above (the rightmost angle labeled θ), then θ 6.9. An angle θ measured in degrees is called obtuse if 90 <θ<80. The Area of a Polygon One way to find the area of a polygon is to decompose the polygon into triangles and then compute the sum of the areas of the triangles. This procedure works particularly well for a regular polygon, which is a polygon all of whose sides have the same length and all of whose angles are equal. For example, a regular polygon with four sides is a square. As another example, the figure here shows a regular octagon inscribed inside a circle. The following example illustrates the procedure for finding the area of a regular polygon. Find the area of a regular octagon whose vertices are eight equally spaced points on the unit circle. example The figure here shows how the octagon can be decomposed into triangles by drawing line segments from the center of the circle (the origin) to the vertices. Each triangle shown here has two sides that are radii of the unit circle; thus those two sides of the triangle each have length. The angle between those two radii is radians (because one rotation around the entire circle is an angle of π radians, π 8 and each of the eight triangles has an angle that takes up one-eighth of the total). Now π 8 radians equals π radians (or 5 ). Thus each of the eight triangles has area sin π, which equals. Thus the sum of the areas of the eight triangles equals 8, which equals. In other words, the octagon has area. Once we know the area of a regular octagon inscribed in the unit circle, we can find the area of a regular octagon of any size. The idea is first to find the length of each side of a regular octagon inscribed in the unit circle, then scale appropriately, remembering that area is proportional to the square of side lengths. The example below illustrates this procedure. (a) Find the length of each side of a regular octagon whose vertices are eight equally spaced points on the unit circle. Find the area of a regular octagon with sides of length s. example 5 (a) Suppose one of the vertices of the regular octagon is the point (, 0), as shown in the figure above. If we move counterclockwise along the unit circle, the next vertex is the point ( cos π 8, sin π ) ( 8, which equals, ). Thus the length of

7 6 chapter 6 Applications of Trigonometry This result implies that a regular octagon whose vertices are equally spaced points on the unit circle has perimeter 8. each side of this regular octagon equals the distance between (, 0) and (, ), which equals ( ) ( + ). Simplifying the expression above, we conclude that each side of this regular octagon has length. The Area Stretch Theorem (see Section.) implies that there is a constant c such that a regular octagon with sides of length s has area cs. From the previous example and from part (a) of this example, we know that the area equals if s =. Thus = c( ) = c( ). Solving this equation for c, we have c = = + + = +. Thus a regular octagon with sides of length s has area ( + )s. exercises Most coins are round, but a few countries have coins that are regular polygons. The picture in the margin shows the one-dollar Canadian coin, which is an -sided regular polygon. The techniques used in the example above will allow you to compute the area of a face of this coin, as you are asked to do in Exercise 8.. Find the area of a triangle that has sides of length and, with an angle of 7 between those sides.. Find the area of a triangle that has sides of length and 5, with an angle of between those sides.. Find the area of a triangle that has sides of length and 7, with an angle of radians between those sides.. Find the area of a triangle that has sides of length 5 and 6, with an angle of radians between those sides. For Exercises 5 use the following figure (which is not drawn to scale): Θ a 5. Find the value of b if a =, θ = 0, and the area of the triangle equals Find the value of a if b = 5, θ = 5, and the area of the triangle equals Find the value of a if b = 7, θ = π, and the area of the triangle equals Find the value of b if a = 9, θ = π, and the area of the triangle equals. 9. Find the value of θ (in radians) if a = 7, b = 6, the area of the triangle equals 5, and θ< π. b

8 section 6. Using Trigonometry to Compute Area 6 0. Find the value of θ (in radians) if a = 5, b =, the area of the triangle equals, and θ< π.. Find the value of θ (in degrees) if a = 6, b =, the area of the triangle equals 5, and θ>90.. Find the value of θ (in degrees) if a = 8, b = 5, and the area of the triangle equals, and θ>90.. Find the area of a parallelogram that has pairs of sides of lengths 6 and 9, with an angle of 8 between two of those sides.. Find the area of a parallelogram that has pairs of sides of lengths 5 and, with an angle of 8 between two of those sides. 5. Find the area of a parallelogram that has pairs of sides of lengths and 0, with an angle of π 6 radians between two of those sides. 6. Find the area of a parallelogram that has pairs of sides of lengths and, with an angle of π radians between two of those sides. For Exercises 7, use the following figure (which is not drawn to scale except that u is indeed meant to be an acute angle and ν is indeed meant to be an obtuse angle): b u a b 7. Find the value of b if a =, ν = 5, and the area of the parallelogram equals Find the value of a if b = 6, ν = 0, and the area of the parallelogram equals. 9. Find the value of a if b = 0, u = π, and the area of the parallelogram equals Find the value of b if a = 5, u = π, and the area of the parallelogram equals 9.. Find the value of u (in radians) if a =, b =, and the area of the parallelogram equals 0.. Find the value of u (in radians) if a =, b = 6, and the area of the parallelogram equals 9. Ν a. Find the value of ν (in degrees) if a = 6, b = 7, and the area of the parallelogram equals.. Find the value of ν (in degrees) if a = 8, b = 5, and the area of the parallelogram equals. 5. What is the largest possible area for a triangle that has one side of length and one side of length 7? 6. What is the largest possible area for a parallelogram that has pairs of sides with lengths 5 and 9? 7. Sketch the regular hexagon whose vertices are six equally spaced points on the unit circle, with one of the vertices at the point (, 0). 8. Sketch the regular dodecagon whose vertices are twelve equally spaced points on the unit circle, with one of the vertices at the point (, 0). [A dodecagon is a twelve-sided polygon.] 9. Find the coordinates of all six vertices of the regular hexagon whose vertices are six equally spaced points on the unit circle, with (, 0) as one of the vertices. List the vertices in counterclockwise order starting at (, 0). 0. Find the coordinates of all twelve vertices of the dodecagondodecagon whose vertices are twelve equally spaced points on the unit circle, with (, 0) as one of the vertices. List the vertices in counterclockwise order starting at (, 0).. Find the area of a regular hexagon whose vertices are six equally spaced points on the unit circle.. Find the area of a regular dodecagon whose vertices are twelve equally spaced points on the unit circle.. Find the perimeter of a regular hexagon whose vertices are six equally spaced points on the unit circle.. Find the perimeter of a regular dodecagondodecagon whose vertices are twelve equally spaced points on the unit circle. 5. Find the area of a regular hexagon with sides of length s. 6. Find the area of a regular dodecagon with sides of length s.

9 6 chapter 6 Applications of Trigonometry 7. Find the area of a regular -sided polygon whose vertices are equally spaced points on a circle of radius. 8. The face of a Canadian one-dollar coin is a regular -sided polygon (see the picture just before the start of these exercises). The distance from the center of this polygon to one of the vertices is.5 centimeters. Find the area of the face of this coin. problems Some problems require considerably more thought than the exercises. Unlike exercises, problems usually have more than one correct answer. 9. What is the area of a triangle whose sides all have length r? 0. Explain why there does not exist a triangle with area 5 having one side of length and one side of length 7.. Show that if a triangle has area R, sides of length A, B, and C, and angles a, b, and c, then R = 8 A B C (sin a)(sin b)(sin c). [Hint: Write three formulas for the area R, and then multiply these formulas together.]. Find numbers b and c such that an isosceles triangle with sides of length b, b, and c has perimeter and area that are both integers.. Explain why the to Exercise is somewhat close to π.. Use a calculator to evaluate numerically the exact you obtained to Exercise. Then explain why this number is somewhat close to π. 5. Explain why a regular polygon with n sides whose vertices are n equally spaced points on the unit circle has area n sin π n. 6. Explain why the result stated in the previous problem implies that sin π n π n for large positive integers n. 7. Choose three large values of n, and use a calculator to verify that sin π n π n for each of those three large values of n. 8. Show that each edge of a regular polygon with n sides whose vertices are n equally spaced points on the unit circle has length cos π n. 9. Explain why a regular polygon with n sides, each with length s, has area n sin π n ( cos π n ) s. 50. Verify that for n =, the formula given by the previous problem reduces to the usual formula for the area of a square. 5. Explain why a regular polygon with n sides whose vertices are n equally spaced points on the unit circle has perimeter n cos π n. 5. Explain why the result stated in the previous problem implies that n cos π n π for large positive integers n. 5. Choose three large values of n, and use a calculator to verify that n cos π n π for each of those three large values of n. 5. Show that cos π n π n if n is a large positive integer.

11 66 chapter 6 Applications of Trigonometry A calculator shows that this is approximately Find the area of a parallelogram that has pairs of sides of lengths and 0, with an angle of π 6 radians between two of those sides. The area of this parallelogram equals 0 sin π, which equals 0. 6 For Exercises 7, use the following figure (which is not drawn to scale except that u is indeed meant to be an acute angle and ν is indeed meant to be an obtuse angle): b u a b 7. Find the value of b if a =, ν = 5, and the area of the parallelogram equals 7. Because the area of the parallelogram equals 7, we have 7 = ab sin ν = b sin 5 = b. Solving the equation above for b, weget b = 7 = Find the value of a if b = 0, u = π, and the area of the parallelogram equals 7. Ν a u = sin Find the value of ν (in degrees) if a = 6, b = 7, and the area of the parallelogram equals. Because the area of the parallelogram equals, we have = ab sin ν = 6 7 sin ν = sin ν. Solving the equation above for sin ν, we get sin ν =. Because ν is an obtuse angle, we thus have ν = π sin radians. Converting this to degrees, we have ν = 80 (sin.. ) 80 π 5. What is the largest possible area for a triangle that has one side of length and one side of length 7? In a triangle that has one side of length and one side of length 7, let θ denote the angle between those two sides. Thus the area of the triangle will equal sin θ. We need to choose θ to make this area as large as possible. The largest possible value of sin θ is, which occurs when θ = π (or θ = 90 if we are working in degrees). Thus we choose θ = π, which gives us a right triangle with sides of length and 7 around the right angle. Because the area of the parallelogram equals 7, we have 7 = ab sin u = 0a sin π = 5a. Solving the equation above for a, weget a = 7 5 = This right triangle has area, which is the largest area of any triangle with sides of length and 7.. Find the value of u (in radians) if a =, b =, and the area of the parallelogram equals 0. Because the area of the parallelogram equals 0, we have 7. Sketch the regular hexagon whose vertices are six equally spaced points on the unit circle, with one of the vertices at the point (, 0). 0 = ab sin u = sin u = sin u. Solving the equation above for sin u, weget sin u = 5 6. Thus

13 68 chapter 6 Applications of Trigonometry 6. The Law of Sines and the Law of Cosines section objectives By the end of this section you should be able to use the law of sines; be able to use the law of cosines; understand when to use which of these two laws. In this section we will learn how to find all the angles and the lengths of all the sides of a triangle given only some of this data. The Law of Sines A c b B a C The lengths of the sides of the triangle shown here have been labeled a, b, and c. The angle opposite the side with length a has been labeled A, the angle opposite the side with length b has been labeled B, and the angle opposite the side with length c has been labeled C. We know from the last section that the area of the triangle equals onehalf the product of the lengths of any two sides times the sine of the angle between those two sides. Different choices of the two sides of the triangle will lead to different formulas for the area of the triangle. As we are about to see, setting those different formulas for the area equal to each other leads to an interesting result. Using the sides with lengths b and c, we see that the area of the triangle equals bc sin A. Using the sides with lengths a and c, we see that the area of the triangle equals ac sin B. Using the sides with lengths a and b, we see that the area of the triangle equals ab sin C. Setting the three formulas obtained above for the area of the triangle equal to each other, we get bc sin A = ac sin B = ab sin C. Multiplying all three expressions above by and then dividing all three expressions by abc gives a result called the law of sines:

14 section 6. The Law of Sines and the Law of Cosines 69 Law of sines sin A a = sin B b = sin C c in a triangle with sides whose lengths are a, b, and c, with corresponding angles A, B, and C opposite those sides. Using the Law of Sines The following example shows how the law of sines can be used to find the lengths of all three sides of a triangle given only two angles of the triangle and the length of one side. Find the lengths of all three sides of the triangle shown here in the margin. example Applying the law of sines to this triangle, we have sin 76 = sin 6. b c 76 b Solving for b, weget sin 6 b = sin 76.67, where the approximate value for this was obtained with the use of a calculator. To find the length c, we will want to apply the law of sines. Thus we first find the angle C. We have C = =. 6 C Now applying the law of sines again to the triangle above, we have Solving for c, we get sin 76 = sin. c sin c = sin When using the law of sines, sometimes the same ambiguity arises as we saw in the last section, as illustrated in the following example. Find all the angles in a triangle that has one side of length 8, one side of length 5, and an angle of 0 opposite the side of length 5. example Labeling the triangle as on the first page of this section, we take b = 8, c = 5, and C = 0. Applying the law of sines, we have sin B 8 = sin 0. 5 Using the information that sin 0 =, we can solve the equation above for sin B, getting

15 70 chapter 6 Applications of Trigonometry sin B = 5. Now sin 5, when converted from radians to degrees, is approximately 5, which suggests that B 5. However, 80 minus this angle also has a sine equal to 5, which suggests that B 7. There is no way to distinguish between these two choices, which are shown below, unless we have some additional information (for example, we might know that B is an obtuse angle, and in that case we would choose B 7 ) Both these triangles have one side of length 8, one side of length 5, and an angle of 0 opposite the side of length 5. Once we decide between the two possible choices of approximately 5 or 7 for the angle opposite the side of length 8, the other angle in the triangle is forced upon us by the requirement that the sum of the angles in a triangle equals 80. Thus if we make the choice on the left above, then the unlabeled angle is approximately 97, but if we make the choice on the right, then the unlabeled angle is approximately. The law of sines does not always lead to an ambiguity when given the lengths of two sides of a triangle and the angle opposite one of the sides, as shown in the following example. example Find all the angles in a triangle that has one side of length 5, one side of length 7, and an angle of 00 opposite the side of length 7. A A triangle that has one side of length 5, one side of length 7, and an angle of 00 opposite the side of length 7 must look like this, where A 5. and B.7. B Labeling the angles of the triangle as shown here and applying the law of sines, we have sin B sin 00 =. 5 7 Thus 5 sin 00 sin B = Now sin 0.70, when converted from radians to degrees, is approximately.7, which suggests that B.7. Note that 80 minus this angle also has a sine equal to 0.70, which suggests that B 5. might be another possible choice for B. However, that choice would give us a triangle with angles of 00 and 5., which adds up to more than 80. Thus this second choice is not possible. Hence there is no ambiguity here we must have B.7. Because = 5., the third angle of the triangle is approximately 5.. Once we know all three angles of the triangle, we could use the law of sines to find the lengths of the other two sides.

16 section 6. The Law of Sines and the Law of Cosines 7 The Law of Cosines The law of sines is a wonderful tool for finding the lengths of all three sides of a triangle when we know two of the angles of the triangle (which means that we know all three angles) and the length of at least one side of the triangle. Also, if we know the lengths of two sides of a triangle and one of the angles other than the angle between those two sides, then the law of sines allows us to find the other angles and the length of the other side, although it may produce two possible choices rather than a unique. However, the law of sines is of no use if we know the lengths of all three sides of a triangle and want to find the angles of the triangle. Similarly, the law of sines cannot help us if the only information we know about a triangle is the length of two sides and the angle between those sides. Fortunately the law of cosines, our next topic, provides the necessary tools for these tasks. Consider a triangle with sides of lengths a, b, and c and an angle of C opposite the side of length c, as shown here. Drop a perpendicular line segment from the vertex opposite the side of length b to the side of length b, as shown above. The length of this line segment is the height of the triangle; label it h. The endpoint of this line segment of length h divides the side of the triangle of length b into two smaller line segments, which we have labeled r and t above. The line segment of length h shown above divides the original larger triangle into two smaller right triangles. Looking at the right triangle on the right, we see that sin C = h a. Thus c r b h t a C As we will see, the law of cosines is a generalization to all triangles of the Pythagorean Theorem, which applies only to right triangles. h = a sin C. Furthermore, looking at the same right triangle, we see that cos C = t a. Thus t = a cos C. The figure above also shows that r = b t. Using the equation above for t, we thus have r = b a cos C. For convenience, we now redraw the figure above, replacing h, t, and r with the values we have just found for them. c b a cos C a sin C b a a cos C In the figure above, consider the right triangle on the left. This right triangle has a hypotenuse of length c and sides of length a sin C and b a cos C. By the Pythagorean Theorem, we have C

17 7 chapter 6 Applications of Trigonometry c = (a sin C) + (b a cos C) = a sin C + b ab cos C + a cos C = a (sin C + cos C) + b ab cos C = a + b ab cos C. Thus we have shown that c = a + b ab cos C. This result is called the law of cosines. Law of cosines c = a + b ab cos C in a triangle with sides whose lengths are a, b, and c, with an angle of C opposite the side with length c. This reformulation allows use of the law of cosines regardless of the labels used for sides and angles. The law of cosines can be restated without symbols as follows: In any triangle, the length squared of one side equals the sum of the squares of the lengths of the other two sides minus twice the product of those two lengths times the cosine of the angle opposite the first side. Suppose we have a right triangle, with hypotenuse of length c and sides of lengths a and b. In this case we have C = π (or C = 90 if we want to work in degrees). Thus cos C = 0. Hence the law of cosines in this case becomes c a c = a + b, b which is the familiar Pythagorean Theorem. For a right triangle, the law of cosines reduces to the Pythagorean Theorem. Using the Law of Cosines The following example shows how the law of cosines can be used to find all three angles of a triangle given only the lengths of the three sides. The idea is to use the law of cosines to solve for the cosine of each angle of the triangle. Unlike the situation that sometimes arises with the law of sines, there will be no ambiguity because no two angles between 0 radians and π radians (or between 0 and 80 if we work in degrees) have the same cosine. example B 6 5 Find all three angles of the triangle shown here in the margin.. Here we know that the triangle has sides of lengths 5, 6, and 7, but we do not know any of the angles. The angles have been labeled in this figure. Applying the law of cosines, we have A 7 C 6 = cos C.

18 section 6. The Law of Sines and the Law of Cosines 7 Solving the equation above for cos C, weget cos C = 9 5. Thus C = cos 9, which is approximately radians (or, equivalently, approximately 57. ). 5 Now we apply the law of cosines again, this time focusing on the angle B, getting 7 = cos B. Solving the equation above for cos B, weget cos B = 5. Thus B = cos, which is approximately.7 radians (or, equivalently, approximately 78.5 ). 5 To find the third angle A, we could simply subtract from π (or from 80 if we are using degrees) the sum of the other two angles. But as a check that we have not made any errors, we will instead use the law of cosines again, this time focusing on the angle A. We have 5 = cos A. Solving the equation above for cos A, weget cos A = 5 7. Thus A = cos 5, which is approximately radians (or, equivalently, approximately. ). 7 As a check, we can add up our approximate s in angles. Because = 80, all is well. The next example shows how the law of cosines can be used to find the lengths of all the sides of a triangle given the lengths of two sides and the angle between them. Find the lengths of all three sides of the triangle shown here in the margin. Here we know that the triangle has sides of lengths and 5 and that the angle between them equals 0. The side opposite that angle has been labeled c. By the law of cosines, we have c = cos 0. example 5 c 0 5 Thus c = 0 cos 0.. Now that we know the lengths of all three sides of the triangle, we could use the law of cosines twice more to find the other two angles, using the same procedure as in the last example.

19 7 chapter 6 Applications of Trigonometry When to Use Which Law A triangle has three angles and lengths corresponding to three sides. If you know some of these six pieces of data, you can often use either the law of sines or the law of cosines to determine the remainder of the data about the triangle. To determine which law to use, think about how to come up with an equation that has only one unknown: If you know only the lengths of the three sides of a triangle, then the law of sines is not useful because it involves two angles, both of which will be unknown. Thus if you know only the lengths of the three sides of a triangle, use the law of cosines. If you know only the lengths of two sides of a triangle and the angle between them, then any use of the law of sines leads to an equation with either an unknown side and an unknown angle or an equation with two unknown angles. Either way, with two unknowns you will not be able to solve the equation; thus the law of sines is not useful in this situation. Hence use the law of cosines if you know the lengths of two sides of a triangle and the angle between them. The term law is unusual in mathematics. The law of sines and law of cosines could have been called the sine theorem and cosine theorem. Sometimes you have enough data so that either the law of sines or the law of cosines could be used. In such cases, use whichever law will give less ambiguity: Suppose you start by knowing the lengths of all three sides of a triangle. The only possibility in this situation is first to use the law of cosines to find one of the angles. Then, knowing the lengths of all three sides of the triangle and one angle, you could use either the law of cosines or the law of sines to find another angle. However, the law of sines may lead to two choices for the angle rather than a unique choice; thus it is better to use the law of cosines in this situation. Another potential case where you could use either law is when you know the length of two sides of a triangle and an angle other than the angle between those two sides. With the notation from the beginning of this section, this might mean that we know a, c, and C. We could use the law of cosines in the form c = a + b ab cos C to solve for b. However the equation above, where we are thinking of b as the unknown, is a quadratic equation that may have two s that are positive numbers. Thus in this situation it is better to use the law of sines, which will give a unique for b. The box below summarizes when to use which law. As usual, you will be better off understanding how these guidelines arise (you can then always reconstruct them) rather than memorizing them. If you know two angles

20 section 6. The Law of Sines and the Law of Cosines 75 of a triangle, then finding the third angle is easy because the sum of the angles of a triangle equals π radians (or equivalently 80 if we are working in degrees). When to use which law Use the law of cosines if you know the lengths of all three sides of a triangle; the lengths of two sides of a triangle and the angle between them. Use the law of sines if you know two angles of a triangle and the length of one side; the length of two sides of a triangle and an angle other than the angle between those two sides. exercises In Exercises 6 use the following figure (which is not drawn to scale). When an exercise requests that you evaluate an angle, give answers in both radians and degrees. A c. Suppose a = 6, B = 5, and C = 0. Evaluate: (a) A b (c) c. Suppose a = 7, B = 50, and C = 5. Evaluate: (a) A b (c) c. Suppose a = 6, A = π 7 radians, and B = π 7 radians. Evaluate: (a) C b (c) c. Suppose a =, B = π radians. Evaluate: b B a C radians, and C = π (a) A b (c) c 5. Suppose a =, b = 5, and c = 6. Evaluate: (a) A B (c) C 6. Suppose a =, b = 6, and c = 7. Evaluate: (a) A B (c) C 7. Suppose a = 5, b = 6, and c = 9. Evaluate: (a) A B (c) C 8. Suppose a = 6, b = 7, and c = 8. Evaluate: (a) A B (c) C 9. Suppose a =, b =, and C = 7. Evaluate: (a) c A (c) B 0. Suppose a = 5, b = 7, and C =. Evaluate: (a) c A (c) B. Suppose a =, b =, and C = radian. Evaluate: (a) c A (c) B. Suppose a =, b = 5, and C = radians. Evaluate: (a) c A (c) B. Suppose a =, b =, and B = 0. Evaluate: (a) A (assume that A<90 ) C (c) c

21 76 chapter 6 Applications of Trigonometry. Suppose a =, b =, and B = 60. Evaluate: (a) A (assume that A<90 ) (c) C c 5. Suppose a =, b =, and B = 0. Evaluate: (a) A (assume that A>90 ) (c) C c [Exercises 5 and 6 should be compared with Exercises and.] 6. Suppose a =, b =, and B = 60. Evaluate: (a) A (assume that A>90 ) (c) C c problems 7. Write the law of sines in the special case of a right triangle. 8. Show how the previous problem gives the familiar characterization of the sine of an angle in a right triangle as the length of the opposite side divided by the length of the hypotenuse. 9. Show how Problem 7 gives the familiar characterization of the tangent of an angle in a right triangle as the length of the opposite side divided by the length of the adjacent side. 0. Suppose a triangle has sides of length a, b, and c satisfying the equation a + b = c. Show that this triangle is a right triangle.. Show that in a triangle whose sides have lengths a, b, and c, the angle between the sides of length a and b is an acute angle if and only if a + b >c.. Show that r p = cos θ in an isosceles triangle that has two sides of length p, an angle of θ between these two sides, and a third side of length r.. Use the law of cosines to show that if a, b, and c are the lengths of the three sides of a triangle, then c >a + b ab.. Use the previous problem to show that in every triangle, the sum of the lengths of any two sides is greater than the length of the third side. 5. Suppose you need to walk from a point P to a point Q. You can either walk in a line from P to Q, or you can walk in a line from P to another point R and then walk in a line from R to Q. Use the previous problem to determine which of these two paths is shorter. 6. Suppose you are asked to find the angle C formed by the sides of length and in a triangle whose sides have length,, and 7. (a) Show that in this situation the law of cosines leads to the equation cos C =. There is no angle whose cosine equals. Thus part (a) seems to give a counterexample to the law of cosines. Explain what is happening here. 7. The law of cosines is stated in this section using the angle C. Using the labels of the triangle just before Exercise, write two versions of the law of cosines, one involving the angle A and one involving the angle B. 8. Use one of the examples from this section to show that 9. Show that cos 5 + cos cos 9 5 = π. a(sin B sin C) + b(sin C sin A) + c(sin A sin B) = 0 in a triangle with sides whose lengths are a, b, and c, with corresponding angles A, B, and C opposite those sides.

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