improper integral

[meson]

definite integral of (1-x)*e^(-x)dx from 1 to infinity.

i got the integral of (1-x)*e^(-x) to be e^(-x)*x and not quite sure where to go from there, but what i tried is to take the limit as A goes to infinity of e^(-x)*x from A to 1 but i got 0. Not sure what i did wrong, any help is appreciated.

 

[pka]

definite integral of (1-x)*e^(-x)dx from 1 to infinity. i got the integral of (1-x)*e^(-x) to be e^(-x)*x and not quite sure where to go from there, but what i tried is to take the limit as A goes to infinity of e^(-x)*x from A to 1 but i got 0. Not sure what i did wrong, any help is appreciated.

Have a look at this webpage.

 

[meson]

yeah i know the answer i just dont know how to get to it. wolframalpha doesnt show the steps for definite integrals.

 

[pka]

yeah i know the answer i just dont know how to get to it. wolframalpha doesnt show the steps for definite integrals.

You don't know how to evaluate \(\displaystyle \displaystyle\lim _{b \to \infty } \left. {\frac{x}{{e^x }}} \right|_1^b ~???\)

 

[meson]

alright i got it thanks.... i didn't rewrite it like that so i didn't realize i had to use lhoptials.