Answer
$\frac{u}{\sqrt {1-u^2}}$
Work Step by Step
1. Let $sin^{-1}u=t, -\frac{\pi}{2}\le t\le \frac{\pi}{2}$, we have $sin(t)=u$.
2. Let $y=u,r=1$, we have $x=\sqrt {1-u^2}$
3. Thus $tan(sin^{-1}u)=tan(t)=\frac{y}{x}=\frac{u}{\sqrt {1-u^2}}$
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