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« Trigonometric Equations
Trigonometry: (lesson 3 of 3)

Law of Cosines

The law of cosines is used:

1. to find the third side of a triangle given two sides and the included angle.

2. to find an angle when given all 3 sides.

law of cosines

1. We are given a, c, and <B.

law of cosines example

2. We are given side b, c, and <A.

law of cosines example

3. We are given b, a, and <C.

law of cosines example

Example 1:

In triangle ABC, side a= 8 cm, c = 10 cm, and the angle at B is 60. Find side b, angle A and angle C.

law of cosines example

Solution:

1. Side b:

law of cosines solution

2. Angle A

law of cosines solution

3. Angle C

law of cosines solution