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