Ans: (a) $9r^2(r-2s)$ (b) $(r-2s)(3r+s)(3r-s)$
-
$\begin{array}{cl}
& 9r^3 – 18r^2s \\
= & 9r^2(r – 2s)
\end{array}$ -
$\begin{array}{cl}
& 9r^3 – 18r^2s – rs^2 + 2s^3 \\
= & 9r^2(r-2s) – rs^2 + 2s^3 \\
= & 9r^2(r – 2s) -s^2(r – 2s) \\
= & (r-2s)(9r^2 – s^2) \\
= & (r-2s)[(3r)^2 – s^2] \\
= & (r-2s)(3r+s)(3r-s)
\end{array}$
