Question
Expand $: (2a + 5b + 7c)^2$
✓
Answer
$(2a + 5b + 7c)^2 $
$= [(2a) + (-5b) + (-7c)]^2$
$= (2a)^2 + (-5b)^2 + (-7c)^2 + 2(2a)(-5b) + 2(-5b)(-7c) + 2(2a)(-7c)$
$= 4a^2 + 25b^2 + 49c^2 - 20ab + 70bc - 28ac$
$= [(2a) + (-5b) + (-7c)]^2$
$= (2a)^2 + (-5b)^2 + (-7c)^2 + 2(2a)(-5b) + 2(-5b)(-7c) + 2(2a)(-7c)$
$= 4a^2 + 25b^2 + 49c^2 - 20ab + 70bc - 28ac$
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