# Commuting Operators

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1. Apr 20, 2016

### Raptor112

1. The problem statement, all variables and given/known data
It is known that $M_1,M_2, M_3$ commute with each other but I don't see how the second line is achieved even though it says that it's using that $M_1$ and $M_2$ commute?

2. Apr 20, 2016

### TSny

Note M2(1+M1) = M2 + M2M1

How can you rewrite the right hand side using the fact that M1 and M2 commute?

3. Apr 20, 2016

### Raptor112

$M_2 + M_1M_2$

4. Apr 20, 2016

### TSny

OK. Can you see something you can do with this expression that will help?

5. Apr 21, 2016

### Raptor112

$(1 +M_1)M_2$ ?

6. Apr 21, 2016

Yes.