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