Question:
$81 x^{4}-y^{4}$
Solution:
$81 x^{4}-y^{4}$
$=\left(9 x^{2}\right)^{2}-\left(y^{2}\right)^{2}$
$=\left(9 x^{2}+y^{2}\right)\left(9 x^{2}-y^{2}\right) \quad\left[a^{2}-b^{2}=(a+b)(a-b)\right]$
$=\left(9 x^{2}+y^{2}\right)\left[(3 x)^{2}-y^{2}\right]$
$=\left(9 x^{2}+y^{2}\right)(3 x+y)(3 x-y) \quad\left[a^{2}-b^{2}=(a+b)(a-b)\right]$
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