(x-3)(x+7)= x^2+7x-3x+21
Did i do this right?
I can't remember how to do these exactly.

-21 at the end because its -3 * 7

a simple test you can do is set x equal to some constant. both sides should be the same value. for example, x = 0. the left side is clearly -21 but right side is +21, which means it's wrong.

it can have false positives, so it's just meant as a simple sanity check. i usually first use 0 then use 1.

This post was edited by carteblanche on May 31 2015 10:26pm

Just remember the acronym! FOIL

And....in case you were never taught this acronym....

Multiply the

First terms
Outer terms
Inner terms
Last terms

(a + b)(c + d)

ac + ad + bc + bd

calling it FOIL always confused me. i know it works, but conceptually it makes more sense for me to think of it as the distributive property. a is distributed to c and d, then b is distributed to c and d, then combine. the terms are in the same order, but it makes more intuitive sense to me.

That is more intuitive because that's actually how it's proved - FOIL is just a shortcut memorization.

when I taught algebra this is how i taught it and it seemed to stay with my students when they took pre calculus afterwords