6.1 LAW OF SINES

6.2 LAW OF COSINES

Copyright © Cengage Learning. All rights reserved. Introduction

2 Introduction

Oblique triangles—triangles that have no right angles.

Law of Sine 1. Two angles and any side (AAS or ASA)

2. Two sides and an angle opposite one of them (SSA)

Law of Cosine

3. Three sides (SSS)

4. Two sides and their included angle (SAS)

3 Introduction

4 Introduction

5 Example 1 – Given Two Angles and One Side—AAS

For the triangle in figure, C = 120, B = 29, andb = 28 feet. Find the remaining angle and sides. Solution: The third angle of the triangle is A = 180 – B – C

= 180 – 29 – 102

= 49. .

6 Example 1 – Solution

The third angle of the triangle is A = 180 – B – C

= 180 – 29 – 102

= 49.

By the Law of Sines, you have

.

7 The Ambiguous Case (SSA)

8 The Ambiguous Case (SSA)

9 Example 3 – Single-Solution Case—SSA

For the triangle in Figure 6.4, a = 22 inches, b = 12 inches, and A = 42. Find the remaining side and angles.

One solution: a  b

Figure 6.4

10 Example 3 – Solution

By the Law of Sines, you have

Reciprocal form

Multiply each side by b.

Substitute for A, a, and b.

B is acute.

11 Example 3 – Solution cont’d

Now, you can determine that

C  180 – 42 – 21.41

= 116.59.

Then, the remaining side is

12 Area of an Oblique Triangle

13 Area of an Oblique Triangle

A is acute. A is obtuse.

Area = (base)(height) = (c)(b sin A) = bc sin A.

14 Area of an Oblique Triangle

15 Example 6 – Finding the Area of a Triangular Lot

Find the area of a triangular lot having two sides of lengths 90 meters and 52 meters and an included angle of 102.

Solution: Consider a = 90 meters, b = 52 meters, and angle C = 102, as shown in figure.

Then, the area of the triangle is

Area = ab sin C

= (90)(52)(sin 102)

 2289 square meters. 16 Example 1 – Three Sides of a Triangle—SSS

Find the three angles of the triangle in Figure 6.11.

Figure 6.11

Solution: It is a good idea first to find the angle opposite the longest side—side b in this case. Using the alternative form of the Law of Cosines, you find that

17 Example 1 – Solution cont’d

Because cos B is negative, you know that B is an obtuse angle given by B  116.80.

At this point, it is simpler to use the Law of Sines to determine A.

18 Example 1 – Solution cont’d

You know that A must be acute because B is obtuse, and a triangle can have, at most, one obtuse angle.

So, A  22.08 and

C  180 – 22.08 – 116.80

= 41.12.

19 Introduction

Do you see why it was wise to find the largest angle first in Example 1? Knowing the cosine of an angle, you can determine whether the angle is acute or obtuse. That is,

cos  > 0 for 0 <  < 90 Acute

cos  < 0 for 90 <  < 180. Obtuse

So, in Example 1, once you found that angle B was obtuse, you knew that angles A and C were both acute.

If the largest angle is acute, the remaining two angles are acute also.

20 Application

21 Example 7 – An Application of the Law of Sines

The course for a boat race starts at point A in Figure 6.9 and proceeds in the direction S 52 W to point B, then in the direction S 40 E to point C, and finally back to A. Point C lies 8 kilometers directly south of point A. Approximate the total distance of the race course.

Figure 6.9 22 Example 7 – Solution

Because lines BD and AC are parallel, it follows that

BCA  CBD.

Consequently, triangle ABC has the measures shown in Figure 6.10.

The measure of angle B is 180 – 52 – 40 = 88.

Using the Law of Sines,

Figure 6.10 23 Example 7 – Solution cont’d

Because b = 8,

and

The total length of the course is approximately

Length  8 + 6.308 + 5.145

=19.453 kilometers. 24