$$a^{2}-b^{2}-c^{2}-2bc$$
$\left(a-b-c\right)\left(a+b+c\right)$
$$a^{2}-\left(b^{2}+c^{2}+2bc\right)$$
$$a^{2}-\left(b+c\right)^{2}$$
$$\left(a-b-c\right)\left(a+b+c\right)$$
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$-\left(b+c\right)^{2}+a^{2}$
