1. Objective To apply the Law of Cosines
Essential Understanding
If you know the measures of two side lengths and the measure of the included angle (SAS), or all three side lengths (SSS), then you can find all the other measures of the triangle.

2. A farmer needs to put a pipe through a hill for irrigation
A farmer needs to put a pipe through a hill for irrigation. The farmer attaches a 14.5 meter rope and an 11.2 meter rope at each entry point of the pipe and makes a triangle. The ends meet at a 580 angle.
What is the length of the pipe the farmer needs?
Can you use Law of Sines?
No, you don’t know the angles that are opposite the sides
Pythagorean Theorem?
Not a right triangle x
11.2 m
14.5 m
580

3. Law of Cosines
For any triangle ABC, the Law of Cosines relates the cosine of each angle to the side lengths of the triangle. C
a2 = b2 + c2 − 2bccosA b c a
b2 = a2 + c2 − 2accosB A B
c2 = a2 + b2 − 2abcosC

4. Using the Law of Cosines (SAS)
Find b to the nearest tenth. 44 22 b 10 B C
b2 = a2 + c2 − 2accosB Law of Cosines
b2 = − 2(22)(10)cos Substitute b b

5. Using the Law of Cosines (SSS)
4.4
7.1
6.7
Find V to the nearest tenth. U V
b2 = a2 + c2 − 2accosB Law of Cosines
4.42 = − 2(6.7)(7.1)cosV Substitute
Solve for angle V.
19.36 = − 95.14cosV Substitute

6. Examples x Law of Cosines c2 = a2 + b2 − 2abcosC
580 x
Law of Cosines
c2 = a2 + b2 − 2abcosC
x2 = − 2(11.2)(14.5)cos580
x2 = 163.6
x = 12.8 m
c2 = a2 + b2 − 2abcosC
42 = − 2(5)(7)cosxo xo 4 7 5
16 = − 70cosxo
-58 = − 70cosxo
.829 = cosxo
W.S. Law of Cosines
34o = x