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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.
You can draw in the lines that form a right-angled triangle, using these points as two of the corners: It's easy to find the lengths of the horizontal and vertical sides of the right triangle: just subtract the x-values and the y-values: Then use the Pythagorean Theorem to find the length of the third side (which is the hypotenuse of the right triangle): c2 = a2 + b2.