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oddiseas
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Homework Statement
This is a past exam question that involves solving a differential equation. I am using a Fourier series. in the solution they simply state that it is obvious that this equals pi^2/4 for n=1, 0 for n is odd>1, and -4n/(n^2-1) for even n.
A{n}=2/pi∫(xsinx)sin nx dx
I always integrate by parts, which for this equation is way too long, especially for an exam. So i am wondering how can i evaluate the value of the integral without integrating? i have tried using the addition formulas, but that seems like just as much work.
Another point of confusion is knowing when a function is even or odd. isn't the function xsinx an even function since f(-X)=-xsin(-X)=-(-sinx)=xsinx and thus an odd function * an even gives an odd, and i thought that the integral would then be zero. But in the solution they specify to use a sine series to fulfill the initial conditions that u(x,t)=u(pi,t)=0.