Homework Help: Solving an integral

1. Mar 31, 2008

fk378

1. The problem statement, all variables and given/known data
Find the integral of x(e^-x) evaluated at 1 to inf

2. Relevant equations
int by parts

3. The attempt at a solution
I tried setting u=e^-x and dv=x dx
So then du=-(e^-x)(-1)=e^-x
and v= (x^2)/2
When plugging in uv - integral (v du)
I get another product for the integral which (x^2)/2 * (e^-x)

How do I get out of this?

2. Mar 31, 2008

Pacopag

Try the other substitution. u=x and dv=e^-xdx.
A good rule of thumb for IBP is to try to choose u to be the part with the "simplest" derivative.
In your case, choosing u=e^-x has du/dx = -e^-x which is not really simpler, but has the same "complexity". But choosing u=x has du/dx=1, which is just a constant (can't get much simpler than that).

3. Mar 31, 2008

fk378

So would the integral (the dv) of e^-x just be e^-x?

4. Mar 31, 2008

Pacopag

almost. Whenever you do an integral (without the limits), you can check if it is right by taking the derivative of your answer. If you are right, then you should get back the function you were integrating.

5. Mar 31, 2008

fk378

Okay, so I just took the derivative of e^-x and I got back e^-x. How come e^-x wouldn't be the antiderivative then?

6. Mar 31, 2008

Pacopag

Don't forget to use the chain rule on the exponent (-x).
$${de^{-x}\over dx}=-e^{-x}$$.

7. Mar 31, 2008

Pacopag

In detail
$${de^{-x}\over dx}={de^u \over du}{du\over dx}$$

where $$u=-x$$.

8. Mar 31, 2008

fk378

Isn't it d/dx (e^-x) = -(e^-x)(-1) = e^-x?

9. Mar 31, 2008

rock.freak667

$$\frac{d}{dx}(e^{f(x)})=f'(x)e^{f(x)}$$

in this case f(x)=-x and hence f'(x)=-1