No worries. Ptolemy's theorem is not something standard here, it was given in a paper (4U once) [We were asked to derive it in each part]. Although if you were to give it straight out in the actual HSC exam you'd probably be still given the marks. However with school teachers - they may not be that lenient (or even know the theorem at all!).
Good luck with your services All the best!
Good luck with your services All the best!