Question

Prove that cos2x=(1-tan^2x)/(1+tan^2x)

I can't find any identities that can help me with this. I'm really confused

Answer 1

1 + tan^2 x= (cos^2 x+ sin^2 x)/cos^2 x = 1/cos^2 x

Therefore
=(1-tan^2x)/(1+tan^2x)= cos^2x - sin^2x
= cos 2x