Use the sine law.
which is true.
I'll write a solution that can definitely be simplified, I'm not happy it uses two other triangle identities, but they are well known and I included links to their proofs below.
First, note that (otherwise ).
Here we go:
'2nd =' is "Law of tangents", see e.g. Law of tangents - Wikipedia, the free encyclopedia
'10th =' is Mollweide's formula, for proof see this thread http://www.mathhelpforum.com/math-he...s-formula.html
First, notice that . So (turning the fractions upside down) what we are trying to prove is equivalent to .
Next, the angles in the triangle are A, B=90º+A and C=90º–2A. Thus and . The sine rule in the triangle then tells you that say. So , and .
But , so . Also, , so . Therefore : just what we wanted!