Answer
$-\frac{4}{3}$
Work Step by Step
Step 1. Letting $cos^{-1}(\frac{4}{5})=u$, we have $cos(u)=\frac{4}{5}$, thus $cot(u)=\frac{4}{3}$
Step 2. We have $sin^{-1}(1)=\frac{\pi}{2}$
Step 3. $tan(u+\frac{\pi}{2})=-cot(u)=-\frac{4}{3}$
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