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In the last lesson we used the Law of Cosines to find the length of the third side of a triangle when you know the length of two sides and the angle in between. This law can also be used to find the measure of an angle when the three sides of a triangle are given. To use the Law of Cosines to find an unknown angle: 1. Identify c as the side across from the angle you are trying to find. 2. Substitute the values into the Law of Cosines.
Using Other Methods. 1  1. Find the third angle of an isosceles triangle. Isosceles triangles have two equal sides and two equal angles. 2  2. Find the third angle of an equilateral triangle. An equilateral triangle has all equal sides and all equal angles. 3  3. Find the third angle of a right triangle.