How do you factor 3x^2 + 5x - 12?

1 Answer
May 30, 2015

Use a version of the AC Method.

A=3, B=5, C=12

Since the sign on the constant term is -, look for a pair of factors of AC=36 whose difference is B=5

The pair 4, 9 works.

Now use that pair to split the middle term, then factor by grouping:

3x^2+5x-12

=3x^2-4x+9x-12

=(3x^2-4x)+(9x-12)

=x(3x-4)+3(3x-4)

=(x+3)(3x-4)