It's me again and I've got another problem that I've been working on for two days, and I just can't get started on it!

Prove the identity (1 - tan^2x ) / (1 + tan^2x) = 1 - 2sin^2x.

So what I have so far is:

(1 - tan^2x) / (sec^2x)

Now what I do next?