Posted by Amelie on .
I'm having a hard time factoring polynomials, especially 3rd degree.
x^3+2x^2+4x8
x^35x^2+20x+12
I don't know how to begin. I thought long division, but what would I divide by?

Math: Factoring 
drwls,
First find one root by trial and error. Call it a. Then divide (xa) into the cubic using long division to get a quadratic equation. Get the roots of that by the usual method.
For the equation
x^3+2x^2+4x8,
one of the roots is x=2. Divide (x2) into x^3+2x^2+4x8, and you get
x^2 + 4 = (42)(x+2)
That completes the factoring.
x^3+2x^2+4x8 = (x2)(x+2)(2x)