Question

how to get
$$\int_0^\infty \ x^n d\frac{(e^{-ax})}{-a}$$
from
$$\int_0^\infty \ x^n {(e^{-ax})} {dx}$$

Answer 1

http://math.stackexchange.com/questions/496088/how-to-evaluate-int-0-infty-exp-ax2-frac-bx2-dx-for-a-b0 hope this helps

Answer 2

Thank you

Answer 3

do the sub:
\[u = -\frac{e^{−ax}}{a}; \ du = -\frac{1}{a}(-a e^{-ax}) \ dx; \ dx = \frac{ du}{e^{-ax} }\]

and switch the limits round

Answer 4

Thank you!!. It is based on reduction formula