Answer
$\sqrt[12]{x}$
Work Step by Step
$\sqrt[n]{x}=x^{1/n}$
$\sqrt[4]{\sqrt[3]{x}} =(x^{\frac{1}{3}})^{\frac{1}{4}}=x^{\frac{1}{3}\cdot\frac{1}{4}}=x^{\frac{1}{12}}=\sqrt[12]{x}$
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