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## Homework Statement

A particle of mass m is in the ground state of the infinte square well. Suddenly the well expands to twice its original size, the right wall moving from a to 2a- leaving the wave function (momentarily) undisturbed. The energy of the particle is now measured. What will be the expectation value of energy?

*Hint:*__If you find yourself confronted with an infinite series, try ANOTHER method__## Homework Equations

## The Attempt at a Solution

The Hint is what is troubling me. I did encounter an infinite series, but I was able to find its sum using some mathematical manipulations.

But I'm curious to know what is the OTHER method the author is talking about. I thought a lot but I cant find any other method, which will not lead to an infinite series.. Any ideas?

Thanks =)

Edit: The answer I got by explicitly summing the series(which is not what the author wants me to do) is [tex]\hbar\omega[/tex], where [tex]\omega=\frac{\pi^{2}\hbar}{2ma^{2}}[/tex]. I would like to verify this as well.