Answer
$\frac{1}{a^2b^2}$
Work Step by Step
$\frac{a}{b^3}\times\frac{b}{a^3}=\frac{a\times b}{b^3\times a^3}=\frac{ab}{a^3b^3}=\frac{ab\div ab}{a^3b^3\div ab}=\frac{1}{a^2b^2}$
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