Factorise:
$5-20 x^{2}$
$=5\left(1-4 x^{2}\right)$
$=5\left[1^{2}-(2 x)^{2}\right]$
$=5(1+2 x)(1-2 x) \quad\left[a^{2}-b^{2}=(a+b)(a-b)\right]$
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