# Thread: Help with using half angle identity/formula

2. ## Re: Help with using half angle identity/formula

$\sin^2(x) = \frac{1}{2} \left( 1 - \cos 2x\right)$

Hence $\sin^4 2x = (\sin^2 2x)^2 = \left(\frac{1}{2}(1-\cos 4x)\right)^2$ expand and use the identity $\cos^2 y = \frac{1}{2} ( 1 + \cos 2y)$ to get $\cos 8x$

3. ## Re: Help with using half angle identity/formula

Originally Posted by Amer
$\sin^2(x) = \frac{1}{2} \left( 1 - \cos 2x\right)$

Hence $\sin^4 2x = (\sin^2 2x)^2 = \left(\frac{1}{2}(1-\cos 4x)\right)^2$ expand and use the identity $\cos^2 y = \frac{1}{2} ( 1 + \cos 2y)$ to get $\cos 8x$
So what value would I input into the red box?