Answer
$\frac{\sqrt[4] {a^3}}{3}$
Work Step by Step
$\sqrt[4]\frac{a^3}{81} =\frac{\sqrt[4] {a^3}}{\sqrt[4] {81}}=\frac{\sqrt[4] {a^3}}{\sqrt[4] {3^4}}=\frac{\sqrt[4] {a^3}}{3}$
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