How do you foil $(5a+3b)(4a-b)$ ?

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Answer 1 · 2015-08-11T21:36:27

Just keep the right order: First-Outer-Inner-Last

F:
$5a*4a=20a^2$
O:
$5a*(-b)=-5ab$
I:
$3b*4a=12ab$
L:
$3b*(-b)=-3b^2$

Then you add up:

$20a^2-5ab+12ab-3b^2=20a^2+7ab-3b^2$

How do you foil (5a+3b)(4a-b)(5a+3b)(4a-b)?