Question

How do you factor `n^2-9n+20`?

Answer 1

`n^2-9n+20=(n-4)(n-5)`

Here you can use the quadratic formula:
let `p(n)=an^2+bn+c`
`Delta=b^2-4ac`
`n_(1,2)=(-b+-sqrt(Delta))/(2a)`
Then `p(n)=a(n-n_1)(n-n_2)`.

In our case we have `a=1, b=-9, c=20` so
`Delta=1, n_1=4, n_2=5`