Methods for finding $\cos(4x)$ given that $\sin(2x) = \frac{3}{5}$ If $\sin(2x) = \frac{3}{5}$
Find $\cos(4x)$..
I tried by : $\cos(4x)= \cos(2\cdot2x)$..
And $\cos (2\cdot2x) = 1-2\sin^2(2x)$ ..
From it ---- $\cos(4x)=0.28$. 
Is there any other ways ? 

*

*Form the 3-4-5 triangle.

*Reflect it about the "4" side. to get a 5-5-6 triangle.

*apply the cosine law to it.
