Answer
$\frac{2\sqrt 6}{7}$
Work Step by Step
Let $csc^{-1}\frac{7}{5}=t$, we have $csc(t)=\frac{7}{5}, sin(t)=\frac{5}{7}$ and $cos(t)=\sqrt {1-sin^2t}=\frac{2\sqrt 6}{7}$
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