Haha I've got this equation while trying to do your other question

What did you get for in terms of sin and cos ?

Something like (1) isn't it ? (next time, it's better you write the result you got, so that we can check it )

Now, let , which is correct, since it satisfies the initial equation.

, so you can divide both sides of (1) by to get

Then just use the well-known identity to transform into

That's all folks !