Posted by Simon on Sunday, November 8, 2009 at 11:16pm.
Prove that cos2x=(1tan^2x)/(1+tan^2x)
I can't find any identities that can help me with this. I'm really confused

Math  drwls, Monday, November 9, 2009 at 10:22am
1 + tan^2 x= (cos^2 x+ sin^2 x)/cos^2 x = 1/cos^2 x
Therefore
=(1tan^2x)/(1+tan^2x)= cos^2x  sin^2x
= cos 2x