A triangle has sides A, B, and C. Sides A and B are of lengths #9# and #12#, respectively, and the angle between A and B is #pi/3#. What is the length of side C?
You use the Cosine rule for this
For the notation of this question and my diagram.
I always understood that the standard practice of labelling was that capital letters were used for the vertices (angles) and that small letters were used for the sides.