Age 14 to 16 Challenge Level:
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.
Use as practice. The problem does not require learners to decide on
the method to use.
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?