Answer
$\sqrt {\frac{1-x}{2}}$
Work Step by Step
Let $cos^{-1}x=y$, we have $cos(y)=x, $ and
$sin(\frac{1}{2}cos^{-1}x)=sin(\frac{y}{2})=\sqrt {\frac{1-cos(y)}{2}}=\sqrt {\frac{1-x}{2}}$
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