1. 13-6 The Law of Cosines Warm Up Lesson Presentation Lesson Quiz
Holt Algebra 2

2. Objectives
Use the Law of Cosines to find the side lengths and angle measures of a triangle.
Use Heron’s Formula to find the area of a triangle.

3. In the previous lesson, you learned to solve triangles by using the Law of Sines. However, the Law of Sines cannot be used to solve triangles for which side-angle-side (SAS) or side-side-side (SSS) information is given. Instead, you must use the Law of Cosines.

4. To derive the Law of Cosines, draw ∆ABC with altitude BD
To derive the Law of Cosines, draw ∆ABC with altitude BD. If x represents the length of AD, the length of DC is b – x.

5. Write an equation that relates the side lengths of ∆DBC.
a2 = (b – x)2 + h2
Pythagorean Theorem
a2 = b2 – 2bx + x2 + h2
Expand (b – x)2.
In ∆ABD, c2 = x2 + h2. Substitute c2 for x2 + h2.
a2 = b2 – 2bx + c2
a2 = b2 – 2b(c cos A) + c2
In ∆ABD, cos A = or x = cos A. Substitute
c cos A for x.
a2 = b2 + c2 – 2bccos A
The previous equation is one of the formulas for the Law of Cosines.

7. Example 1A: Using the Law of Cosines
Use the given measurements to solve ∆ABC. Round to the nearest tenth.
a = 8, b = 5, mC = 32.2°
Step 1 Find the length of the third side.
c2 = a2 + b2 – 2ab cos C
Law of Cosines
c2 = – 2(8)(5) cos 32.2°
Substitute.
c2 ≈ 21.3
Use a calculator to simplify.
c ≈ 4.6
Solve for the positive value of c.

8. Example 1A Continued
Step 2 Find the measure of the smaller angle, B.
Law of Sines
Substitute.
Solve for sin B.
Solve for m B.

9. Example 1A Continued
Step 3 Find the third angle measure.
mA ° °  180°
Triangle Sum Theorem
mA  112.4°
Solve for m A.

10. Example 1B: Using the Law of Cosines
Use the given measurements to solve ∆ABC. Round to the nearest tenth.
a = 8, b = 9, c = 7
Step 1 Find the measure of the largest angle, B.
b2 = a2 + c2 – 2ac cos B
Law of cosines
92 = – 2(8)(7) cos B
Substitute.
cos B =
Solve for cos B.
m B = Cos-1 (0.2857) ≈ 73.4°
Solve for m B.

11. Example 1B Continued
Use the given measurements to solve ∆ABC. Round to the nearest tenth.
Step 2 Find another angle measure
c2 = a2 + b2 – 2ab cos C
Law of cosines
72 = – 2(8)(9) cos C
Substitute.
cos C =
Solve for cos C.
m C = Cos-1 (0.6667) ≈ 48.2°
Solve for m C.

12. Example 1B Continued
Use the given measurements to solve ∆ABC. Round to the nearest tenth.
Step 3 Find the third angle measure.
m A ° °  180°
Triangle Sum Theorem
m A  58.4°
Solve for m A.

13. Check It Out! Example 1a
Use the given measurements to solve ∆ABC. Round to the nearest tenth.
b = 23, c = 18, m A = 173°

14. Check It Out! Example 1b
Use the given measurements to solve ∆ABC. Round to the nearest tenth.
a = 35, b = 42, c = 50.3

15. The largest angle of a triangle is the angle opposite the longest side.
Remember!

16. Understand the Problem
Example 2: Problem-Solving Application
If a hiker travels at an average speed of 2.5 mi/h, how long will it take him to travel from the cave to the waterfall? Round to the nearest tenth of an hour. 1
Understand the Problem
The answer will be the number of hours that the hiker takes to travel to the waterfall.

17. Understand the Problem1
Understand the Problem
The answer will be the number of hours that the hiker takes to travel to the waterfall.
List the important information:
The cave is 3 mi from the cabin.
The waterfall is 4 mi from the cabin. The path from the cabin to the waterfall makes a 71.7° angle with the path from the cabin to the cave.
The hiker travels at an average speed of 2.5 mi/h.

18. 2
Make a Plan
Use the Law of Cosines to find the distance d between the water-fall and the cave. Then determine how long it will take the hiker to travel this distance.

19. Solve 3
Step 1 Find the distance d between the waterfall and the cave.
d2 = c2 + w2 – 2cw cos D
Law of Cosines
Substitute 4 for c, 3 for w, and 71.7 for D.
d2 = – 2(4)(3)cos 71.7°
d2 ≈ 17.5
Use a calculator to simplify.
d ≈ 4.2
Solve for the positive value of d.

20. Step 2 Determine the number of hours.
The hiker must travel about 4.2 mi to reach the waterfall. At a speed of 2.5 mi/h, it will take the hiker ≈ 1.7 h to reach the waterfall.
Look Back 4
In 1.7 h, the hiker would travel 1.7  2.5 = 4.25 mi. Using the Law of Cosines to solve for the angle gives 73.1°. Since this is close to the actual value, an answer of 1.7 hours seems reasonable.

21. Check It Out! Example 2
A pilot is flying from Houston to Oklahoma City. To avoid a thunderstorm, the pilot flies 28° off the direct route for a distance of 175 miles. He then makes a turn and flies straight on to Oklahoma City. To the nearest mile, how much farther than the direct route was the route taken by the pilot?

22. The Law of Cosines can be used to derive a formula for the area of a triangle based on its side lengths. This formula is called Heron’s Formula.

23. Example 3: Landscaping Application
A garden has a triangular flower bed with sides measuring 2 yd, 6 yd, and 7 yd. What is the area of the flower bed to the nearest tenth of a square yard?
Step 1 Find the value of s.
Use the formula for half of the perimeter.
Substitute 2 for a, 6 for b, and 7 for c.

24. Example 3 Continued
Step 2 Find the area of the triangle.
A =
Heron’s formula
Substitute 7.5 for s.
A =
A = 5.6
Use a calculator to simplify.
The area of the flower bed is 5.6 yd2.

25.  Example 3 Continued Check Find the measure of the largest angle, C.
c2 = a2 + b2 – 2ab cos C
Law of Cosines
72 = – 2(2)(6) cos C
Substitute.
cos C ≈ –0.375
Solve for cos C.
m C ≈ 112.0°
Solve for m c.
Find the area of the triangle by using the formula area = ab sin c.
area 

26. Check It Out! Example 3
The surface of a hotel swimming pool is shaped like a triangle with sides measuring 50 m, 28 m, and 30 m. What is the area of the pool’s surface to the nearest square meter?