Subject: Re: Trigonometric identity
Date: Friday, January 24, 1997 7:45 PM
From: Alex Bogomolny
Regardless of where you live, I am quite uncomfortable with the manner of your question. I'd be much happier if you explained the nature of your difficulty. Say, you tried to solve the problem in such-and-such way but got stuck at such-and-such point. Following is a solution. It's up to you to learn something from it.
Two formulas are needed for the reference sake:
The latter is of course a particular case of the former. Now, multiply both sides of the identity by cos(a)*sin(a). After simplifications, you'll get
or, from 1.,
which is true from 2.