Answer
$sin~(arccos~u) = \sqrt{1-u^2}$
Work Step by Step
Let $~~\theta = arccos~u$
Then $~~cos~\theta = u$
$sin~\theta = \sqrt{1-cos^2~\theta}$
$sin~\theta = \sqrt{1-u^2}$
Therefore, $~~sin~(arccos~u) = \sqrt{1-u^2}$
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