You are allowed to. However, the HSC exam questions are almost always designed so that no one should resort to it, otherwise it would disadvantage the 3U only students. For example, for things like deriving triple angle formulae you would be told explicitly to derive it using some 3U method (compound angles). If you opt for a different method to the method that the question asks for then you will lose marks. If the question was dodgy and didn't specify what method to use to prove the result, then the complex numbers approach would be acceptable.