Hello arzeThe case where is obtuse simply gives:and then using , the result follows immediately.

I'm not sure what is expected if you are to show that each of these is equal to . I should be quite happy to accept the argument:Similarly, by dropping the perpendicular from to :This is much the most straightforward method. However, if you are not allowed to do that, and we are given the formula

then the only thing I can suggest is that we denote and , and then say:But that seems excessively complicated!

Grandad