inverse trig functions

[ijd5000]

The question asks to simplify

cos(2 arctan (x))

the '2' is whats throwing me off, i know it would be 1/sqrt(1+x^2) if it wasn't there.

 

[daon2]

\(\displaystyle \cos(2\cdot(\ddot\smile)) = \cos^2(\ddot\smile) - \sin^2(\ddot\smile)\)

The set up the triangle for \(\displaystyle \tan(\ddot\smile) = \dfrac{x}{1}\) and find the cosine and sine of it.