Question
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$(2 a+5 b+7 c)^2$
$(2 a+5 b+7 c)^2$
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Answer
$(2 a+5 b+7 c)^2$
$=[(2 a)+(-5 b)+(-7 c)]^2$
$=(2 a)^2+(-5 b)^2+(-7 c)^2+2(2 a)(-5 b)+2(-5 b)(-7 c)+2(2 a)(-7 c)$
$=4 a^2+25 b^2+49 c^2-20 a b+70 b c-28 a c$
$=[(2 a)+(-5 b)+(-7 c)]^2$
$=(2 a)^2+(-5 b)^2+(-7 c)^2+2(2 a)(-5 b)+2(-5 b)(-7 c)+2(2 a)(-7 c)$
$=4 a^2+25 b^2+49 c^2-20 a b+70 b c-28 a c$
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