Question:
Factorise:
$x^{2}+y^{2}-z^{2}-2 x y$
Solution:
$x^{2}+y^{2}-z^{2}-2 x y$
$=\left(x^{2}+y^{2}-2 x y\right)-z^{2}$
$=(x-y)^{2}-z^{2} \quad\left[a^{2}-2 a b+b^{2}=(a-b)^{2}\right]$
$=(x-y+z)(x-y-z) \quad\left[a^{2}-b^{2}=(a+b)(a-b)\right]$
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