How to factor the trinomial a^3b-9a^2b^2+20ab

Question

I need to factor the trinomial

 

Answer 1

Well first let's look for monomial factors, things that all the terms share as a factor. In this problem all the terms have at least a^1 and b^1 as a factor. So:

 

(ab) (a^2 - 9ab + 20)

 

Now in this case, that is as far as it can be factored (using rational numbers.)