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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.
If you know the side lengths a, b and c you can find cos(C) and hence the measure of the angle C. Hi Lucy. Here's an example of how I would use Harley's Law of Cosines to find the angles of a triangle. Let's say I am told the sides lengths are 10, 4 and 7. First I should assign reference letters. Let side a = 10, side b = 4 and side c = 7. By convention, the angles (or corners) of the triangle are capital letters corresponding to the side that is across from them. So I have this diagram, labelled appropriately: Start by picking an angle to use the law of cosines for.