Why do this problem?

The first part is an easy application of the cosine rule. The second part requires some algebraic skills and gives practice in proving a formula.

Possible approach

Use as practice. The problem does not require learners to decide on the method to use.

Key questions

Look at the formula to be proved. Write down the cosine formula for the triangles in the diagram using the same notation. Can you see how to manipulate the formulas you have established to give the formula to be proved?