Can anyone figure out the first few steps in this pre-calc problem?

Question

So I can usually do these problems fine, but I cant even figure out where to start with this one, prove that:
(1-tan^2x)/(1+tan^2x)=cos2x
I am sure I'm just missing some simple step that'll be obvious once someone points it out, but can anyone help me through the first couple steps here and I'm sure I could finish it off after that.

Answer 1

Replace tan with sin/cos:

1 - (sin^2x/cos^2x) / 1 + (sin^2x/cos^2x)

multiply top and bottom by cos^2x

cos^2x - sin^2x / cos^2x + sin^2x

Now cos^2x + sin^2x = 1 and cos^2x - sin^2x = cos2x