Answer
$\frac{8}{9}$
Work Step by Step
We know that $\sqrt \frac{64}{81}=\frac{8}{9}$, because $(\frac{8}{9})^{2}=\frac{8}{9}\times\frac{8}{9}=\frac{8\times8}{9\times9}=\frac{64}{81}$.
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