Answer
$y=2-2cos(x)$
Work Step by Step
$y=4sin^2(\frac{x}{2})$
Remember$\;\;\;\;\;sin(\frac{x}{2})=\pm \sqrt{\frac{1-cos(x)}{2}}$
$y=4(\pm \sqrt{\frac{1-cos(x)}{2}})^2$
$y=4 .\frac{1-cos(x)}{2}$
$y=2(1-cos(x))=2-2cos(x)$
$y=2-2cos(x)$