trig
posted by Paul .
How do I solve this?
If cos x= 3/5, find the exact value of cos 2x

have you come across the identity
cos 2x = 2cos^2 x  1
we can just apply this here.
cos 2x = 2(3/5)^2  1
= 7/25 
No I have not come across that before.

then probably
cos2x = cos^2 x  sin^2 x
(I replaced sin^2 x with 1cos^2 x to get the one I used)
Your question fits right into the application of the double and half angle formulas, and without these your question becomes a mess. 
Oh I get it now. thanks for your help.