Question

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

Answer 1

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

Explanation:

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`