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## Homework Statement

I know how to compute the expectation value of an observable. But how does one compute the expectation value of an observable's square?

## Homework Equations

[tex]\langle Q \rangle = \int_{-\infty}^{\infty} \Psi^* \hat{Q} \Psi \; dx[/tex]

[tex]\langle Q^2 \rangle = \int_{-\infty}^{\infty} \Psi^* (\hat{Q} \Psi)^2 \; dx \; ???[/tex]

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