Ans: $ab^3$
$\begin{array}{cl}
& \dfrac{(ab)^3}{a^2} \\
= & \dfrac{a^3b^3}{a^2} \\
= & a^{3-2} b^3 \\
= & ab^3
\end{array}$
$\begin{array}{cl}
& \dfrac{(ab)^3}{a^2} \\
= & \dfrac{a^3b^3}{a^2} \\
= & a^{3-2} b^3 \\
= & ab^3
\end{array}$
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