1. 8-5 Laws of sines and cosines
Chapter 8
8-5 Laws of sines and cosines

2. Objectives
Use the Law of Sines and the Law of Cosines to solve triangles.

3. Laws of sines and cosines
In this lesson, you will learn to solve any triangle.
To do so, you will need to calculate trigonometric
ratios for angle measures up to 180°. You can use a calculator to find these values.

4. Example 1: Finding Trigonometric Ratios for Obtuse Angles
Use your calculator to find each trigonometric ratio. Round to the nearest hundredth.
A. tan 103°
tan 103°  –4.33

5. Check it out!!!
Use a calculator to find each trigonometric ratio. Round to the nearest hundredth.
a. tan 175°
b. cos 92°

6. Law of sines From the diagram, , and
You can use the altitude of a triangle to find a relationship between the triangle’s side lengths.
In ∆ABC, let h represent the length of the altitude from C to
From the diagram, , and

7. Law of sines
You can use another altitude to show that these ratios equal
By solving for h, you find that h = b sin A and h = a sin B. So b sin A = a sin B, and

8. LAW OF SINES
You can use the Law of Sines to solve a triangle if you are given
• two angle measures and any side length
(ASA or AAS) or
• two side lengths and a non-included angle measure (SSA).

9. Example 2A: Using the Law of Sines
Find the measure. Round lengths to the nearest tenth and angle measures to the nearest degree. FG

10. Example 2B: Using the Law of Sines
Find the measure. Round lengths to the nearest tenth and angle measures to the nearest degree
mQ

11. Check it out!!!!
Find the measure. Round lengths to the nearest tenth and angle measures to the nearest degree. NP

12. Check it out!!!
Find the measure. Round lengths to the nearest tenth and angle measures to the nearest degree.
mL

13. Check it out!!!
Find the measure. Round lengths to the nearest tenth and angle measures to the nearest degree. AC

14. Law of cosines
The Law of Sines cannot be used to solve every triangle. If you know two side lengths and the included angle measure or if you know all three side lengths, you cannot use the Law of Sines. Instead, you can apply the Law of Cosines.

15. Law of cosine
You can use the Law of Cosines to solve a triangle if you are given
• two side lengths and the included angle measure (SAS) or
• three side lengths (SSS).

16. Example 3A: Using the Law of Cosines
Find the measure. Round lengths to the nearest tenth and angle measures to the nearest degree. XZ
XZ  53.3

17. Example 3b
Find the measure. Round lengths to the nearest tenth and angle measures to the nearest degree.
mT

18. Check It Out! Example 3a
Find the measure. Round lengths to the nearest tenth and angle measures to the nearest degree DE
DE  6.5

19. Check it out!!
Find the measure. Round lengths to the nearest tenth and angle measures to the nearest degree.
mK

20. Application
A sailing club has planned a triangular racecourse, as shown in the diagram. How long is the leg of the race along BC? How many degrees must competitors turn at point C? Round the length to the nearest tenth and the angle measure to the nearest degree.

21. Application
What if…? Another engineer suggested using a cable attached from the top of the tower to a point 31 m from the base. How long would this cable be, and what angle would it make with the ground? Round the length to the nearest tenth and the angle measure to the nearest degree.
31 m

22. Student guided practice
Do even problems from 1-14 in your book page 573

23. Homework
Do even problems from in your book page 573

24. Closure Today we learned about the law of sines and the law of cosines
Next class we are going to do an in-class project on tessellations