The Cosine Rule

We can use the cosine rule to:

 1.Find a side when it is opposite a known angle and that angle is between two known sides. 2. Find an angle when all three sides are known. 1. We start by labelling the sides a,b,c and the angles A,B,C with sides opposite their respective angles as shown. 2. We start by labelling the sides a,b,c and the angles A,B,C with sides opposite their respective angles as shown.