Expand each of the following, using suitable identities : $(3 a-7 b-c)^{2}$
Expand each of the following, using suitable identities : $(3 a-7 b-c)^{2}$
$(3 a-7 b-c)^{2}$
Using $(x+y+z)^{2}=x^{2}+y^{2}+z^{2}+2 x y+2 y z+2 z x,$ we have
$(3 a-7 b-c)^{2} =(3 a)^{2}+(-7 b)^{2}+(-c)^{2}+2(3 a)(-7 b)+2(-7 b)(-c)+2(-c)(3 a) $
$=9 a^{2}+49 b^{2}+c^{2}+(-42 a b)+(14 b c)+(-6 c a)$
$=9 a^{2}+49 b^{2}+c^{2}-42 a b+14 b c-6 c a $
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