Question

How do you factor the trinomial `x^2 + 3x - 40`?

Answer 1

`x^2+3x-40 = (x+8)(x-5)`

Explanation:

Note that:

`(x+a)(x-b) = x^2+(a-b)x-ab`

So we want to find `a, b > 0` such that `a-b = 3` and `ab=40`

The pair `8, 5` works and hence:

`x^2+3x-40 = (x+8)(x-5)`