# Expected values in infinite square well

 P: 21 Ok...this must sound stupid, because i didn't found answer on the web and on my books...but i am having trouble with the infinite square well. I want to calculate . V(x)=0 for 0<=x<=a $=\frac{2}{a}\int^{a}_{0} x \sin^2(\frac{n\pi}{a}x)dx$ Doing integration by parts i got to: $\frac{2}{a}\left[\frac{a^2}{2n}-\int^a_0\frac{a}{2n}dx\right]=0$ What i am doing wrong? Thank you
 Mentor P: 11,911 I would expect an error in your integration, as the first expression is not zero and the following one is.
P: 70
 Quote by Aikon $=\frac{2}{a}\int^{a}_{0} x \sin^2(\frac{n\pi}{a}x)dx$

set
$u=\frac{n\pi}{a}x$

look up
$\int u \sin^2(u)du$
in a Table of Integrals to find
$\frac{u^2}{4}-\frac{u\sin(2u)}{4}-\frac{cos(2u)}{8}$

P: 21
Expected values in infinite square well

 Quote by tadchem set $u=\frac{n\pi}{a}x$ look up $\int u \sin^2(u)du$ in a Table of Integrals to find $\frac{u^2}{4}-\frac{u\sin(2u)}{4}-\frac{cos(2u)}{8}$
Yeah, i got it...it is easier to use table of integrals.
it gives the expected $<x>=\frac{a}{2}$ for any value of n.
thanks

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