Answer
$sin(arccos~x)=\sqrt {1-x^2}$
Work Step by Step
Let $u=arccos~x~~$ (Range: $0\lt u\lt\pi$)
Then: $x=cos~u$
$sin^2u+cos^2u=1$
$sin^2u=1-cos^2u=1-x^2$
$sin~u=\sqrt {1-x^2}~~$ ($0\lt u\lt\pi$)
$sin(arccos~x)=sin~u=\sqrt {1-x^2}$