Show that satisfies the equation (I know how to do this ). Express this equation in terms of and . (I can do this too ) .
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 !