Using integration by parts, we find that `int x^(n)e^(-x) dx=`

The integral
`int x^n e^-x dx` has to be determined.

Integration by parts gives us the
rule: `int u dv = u*v - int v du`

Let `u = x^n` and `dv = e^-x dx`

  `du = n*x^(n-1) dx`

`v = -e^(-x)`

`int x^n
e^-x dx`

`= x^n*-1*e^-x - int -1*e^-x*n*x^(n-1) dx`

=
`x^n*-1*e^-x + n*int e^-x*x^(n-1) dx`

= `-x^n*e^-x + n*int e^-x*x^(n-1)
dx`

`int e^-x*x^(n-1) dx`

= `-x^(n-1)*e^-x + (n-1)*int
e^-x*x^(n-2) dx`

Substituting this in the original integral

`int x^n*e^-x dx`

= `-x^n*e^-x + n*(-x^(n-1)*e^-x + (n-1)*int
e^-x*x^(n-2) dx)`

= `-x^n*e^-x- n*x^(n-1)*e^-x + n*(n-1)*int e^-x*x^(n-2)
dx`

= `-e^-x*(x^n+n*x^(n-1))+n*(n-1)*int e^-x*x^(n-2) dx`

This can be continued n times to yield the final result.

=
`-e^-x*(x^n+n*x^(n-1)+ n*(n-1)x^(n-2)+...n!)`

The integral...