Question
Factorise:
$3 a^3 b-243 a b^3$

Answer

$3 a^3 b-243 a b^3$
$=3 a b\left(a^2-81 b^2\right)$
$=3 a b\left[(a)^2-(9 b)^2\right]$
$=3 a b(a+9 b)(a-9 b)\left[\therefore a^2-b^2=(a-b)(a+b)\right]$

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