Answer
$-\frac{4}{3}$
Work Step by Step
1. Let $cos^{-1}(\frac{4}{5})=u$ (quadrant I), we have $cos(u)=\frac{4}{5}, cot(u)=\frac{4}{3}$
2. Let $sin^{-1}(1)=v$, we have $v=\frac{\pi}{2}$
3. Thus $tan(u+v)=tan(u+\frac{\pi}{2})=-cot(u)=-\frac{4}{3}$