Thursday, 20 March 2014

calculus - Integration by parts or substitution?



xexdx




One of my friends said substitution , but I can't seem to get it to work.
Otherwise I also tried integration by parts but I'm not getting the same answer as wolfram.



The space in the question seems like it shouldn't take more than 2 lines though. Am I missing something?



Thanks to all the answers below , I messed up in the original question it was actually



xex2dx



With help from the below answers I did the following:




Let u=x2 , then du=2xdx



So rewriting the integral



xeu12xdx



Simplifying yields:



12xeudx




Which in turn yields:



eu2+C



The rest is fairly obvious!


Answer



Definitely by parts, as substitution of x won't get you anywhere.
Let u=ex and dv=ex in udv=uvvdu
and we have




xexdx=xexexdx=xexex+c



As a general tip, you will usually want to use parts if you have an exponential, which doesn't get any nastier as you anti-differentiate, and a polynomial which will disappear after some number of differentiations. A notable exception is when you have something like
xex2dx


Where a u=x2 substitution will cancel out the x coefficient on the exponential.



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