Factorise:
$20 x^{2}-45$
$=5\left(4 x^{2}-9\right)$
$=5\left[(2 x)^{2}-3^{2}\right]$
$=5(2 x+3)(2 x-3) \quad\left[a^{2}-b^{2}=(a+b)(a-b)\right]$
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