# How do you factor -5a ^ { 7} + 20a ^ { 6} - 20a ^ { 5}?

May 15, 2017

$- 5 {a}^{5} {\left(a - 2\right)}^{2}$

#### Explanation:

When factorising, first look for common factors.

$- 5 {a}^{7} + 20 {a}^{6} - 20 {a}^{5}$

Here we have $- 5 {a}^{5}$ as the GCF

$- 5 {a}^{5} \left({a}^{2} - 4 a + 4\right)$

then look to see if the bracket can be factorised separately

we note it is a quadratic, and a perfect square at that

$- 5 {a}^{5} {\left(a - 2\right)}^{2}$