Basic calculation problem with commutators

AI Thread Summary
The discussion revolves around proving that the time derivative of a Hermitian operator A, which commutes with the Hamiltonian H, is zero. The key equation used is derived from the Schrödinger equation, leading to the expression for the time derivative of the expectation value of A. A confusion arises regarding the terms involving the time derivative of the wave function and the application of the commutator, specifically mistaking the anti-commutator for the commutator. The participant realizes their error in sign when applying the properties of inner products. The thread concludes with a clarification of the correct approach to the problem.
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Homework Statement


A is a Hermitian operator which commutes with the Hamiltonian: \left[A,H\right]=AH-HA=0

To be shown: \frac{d}{dt}A=0

Homework Equations



Schrödinger equation: i\hbar\frac{\partial}{\partial t}\psi=H\psi with the Hamilton operator H.

The Attempt at a Solution


I have seen this solution on many sites:

\frac{d}{dt}<A>=\frac{d}{dt}<\psi|A|\psi>=<\psi|\frac{\partial A}{\partial t}|\psi>+<\frac{d\psi}{dt}|A|\psi>+<\psi|A|\frac{d\psi}{dt}>=<\frac{\partial A}{\partial t}>+\frac{1}{i\hbar}<\left[ A, H\right] >=0

I have a problem with this: <\frac{d\psi}{dt}|A|\psi>+<\psi|A|\frac{d\psi}{dt}>=\frac{1}{i\hbar}<\left[ A, H\right] >

Okay, obviously we have from the Schrödinger equation:
H=i\hbar\frac{\partial}{\partial t}
and thus
\frac{\partial}{\partial t}=\frac{1}{i\hbar}H
and thus
<\frac{d\psi}{dt}|A|\psi>+<\psi|A|\frac{d\psi}{dt}>=\frac{1}{i\hbar}(<H\psi|A|\psi>+<\psi|A|H\psi>)=\frac{1}{i\hbar}<\psi|HA+AH|\psi>

But this is not the commutator but the anti-commutator. It is plus and not minus! What did I do wrong here?
 
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Remember <a|ib>=i*<a|b> but <ia|b>=(-i)*<a|b>.
 
Oh, of course!
Thank you.
 
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