How do you factor completely $x^2 + 10x + 21$ ?

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Answer 1 · 2016-09-23T21:46:42

$x^2+10x+21 = (x+7)(x+3)$

Note that $(x+a)(x+b) = x^2+(a+b)x+ab$

So we want to find a pair of factors of $21$ with sum $10$

The pair $7, 3$ works, so we find:

$x^2+10x+21 = (x+7)(x+3)$

How do you factor completely x^2 + 10x + 21x^2 + 10x + 21?