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# integration by substitution problem - what do I substitute? watch

1. Hey,
so I am sitting here doing homework, and I am stuck. As in seriously stuck. I have tried this question for an hour, in a million different ways, and it doesn't work for me. Maybe it just doesn't like me or something
anyway, I need your help with this. Here goes:
use integration by substitution to work out:
∫(x^2)e^(x^3)dx

It's fine with the x^2 part and e^x, but the e^(x^3) just completely kills me and I have no clue as to how to integrate the whole expression.

I'd greatly appreciate any pointers

Thanks, guys.
2. Try u = x^3. When you work out "du" you'll find that the x^2 disappears nicely. Seriously, just work through with that substitution and you'll see it clears up nicely.
3. Do you know what differentiates to? If so, this question can be done easily by inspection.
4. I believe e^(x^3) differentiates to 3(x^2)*(e^(x^3))/8x^3)

also, how do you use latex, this whole bracket and thingy stuff is really annoying and confusing.

Johnny, will try this now. thanks. will report in a minute
5. Id do what JohnnySPal tells you.

and BTW

if

so e^{x^3} doesnt differentiate to 3(x^2)*(e^(x^3))/8x^3)
6. (Original post by Dave_McDougall)
I believe e^(x^3) differentiates to 3(x^2)*(e^(x^3))/8x^3)
No,

So

/edit: humbug, beaten to it. >_>
7. *sexes insparato*
8. so I did what Johnny said and got as an answer:

right?
edit: if I could actually differentiate I could check >.<
9. Err... No...

After the substitution you should get the integral

So after intagration and converting back to x you get:

Unless I'm being thick?
10. (Original post by JohnnySPal)
Err... No...

After the substitution you should get the integral

So after intagration and converting back to x you get:

Unless I'm being thick?
Nope looks right to me.
11. (Original post by Dave_McDougall)
so I did what Johnny said and got as an answer:

right?
edit: if I could actually differentiate I could check >.<

Immediately here you can tell that can't end up to what you'd want becausew you'd have to use the product rule if I recall correctly.
12. (Original post by JohnnySPal)
Err... No...

After the substitution you should get the integral

So after intagration and converting back to x you get:

Unless I'm being thick?
ah, I know what I did wrong!
this I got: ,
and then I made the mistake of saying ,
i.e. thought that

would integrate to

which of course was wrong.
edit: I suck at latex
13. thanks everyone, I can now go to bed a happy chappy! Man, and this is only A2 normal!! how should I survive the FM?!?!

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