# Is this guy for real? Look at this integral result, did I type somthing in wrong?

1. Jan 15, 2006

### mr_coffee

Hello everyone. I'm solving this D.E. and i used maple 9 to figure out the integral of:
(-25*x)/(3*e^(5*x))
it spit out:
(1/3*1/(ln(e)^2)+5/3*1/ln(e)*x)/exp(5*x*ln(e))
Here is what it is, sorry i don't LaTeX it up, its easier for me to just scan:
http://img495.imageshack.us/img495/4138/lastscan1vb.jpg [Broken]

Last edited by a moderator: May 2, 2017
2. Jan 15, 2006

### matt grime

Lets obviously get rid of constants since they don't matter, so you just want to integrate

$$xe^{-5x}$$

which is the same as integrating

$$ye^{5y}$$

if you really want to get rid of the minus sign, and that's straightforward to to by parts.

I don't see why that's sometihng that makes you ask 'if this guy is for real' and require you to use maple.

3. Jan 15, 2006

### mr_coffee

well i'm a mere computer engineering student, ur a mathematics guru! so that integral scares me.

so integrating x*e^(-5*x) is still not pretty:
-1/25*1/ln(e)^2*exp(-5*x*ln(e))-1/5*x/ln(e)*exp(-5*x*ln(e));
Thanks just checking to make sure thats the outcome of the integral.

4. Jan 15, 2006

### Hurkyl

Staff Emeritus
Yes, but this integral (or one almost identical to it) appears as an example in your calc text. :tongue2:

5. Jan 15, 2006

### fasterthanjoao

worth pointing out, I think, is that this thread illustrates exactly why it's useful to learn how to do maths all the maths you require by hand. (not pointing the finger at you or anything johnny, just generally).

Means that you'll know and be assured that your maths proggies are indeed capable. understanding is everything

6. Jan 15, 2006

### matt grime

I haven't done an integral for my own reasons since, ooh, highschool, or close to it. A mathematician is one of the least likely people to be able to do integrals of all the physical scientists. But that one is really easy even for me. Integrals are either easy or impossible (mostly the latter) and since there are only, what 4 things you can do to an integral (do it, parts, trig, hyperbolic trig) then looking at the few options ought to be the first thing you do rather than ask for help straight away. Of course I'm assuming you know what integration by parts is, as I'm sure your teacher did.

Last edited: Jan 15, 2006