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Commuting Operators

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?
2016-04-20 (2).png
 

TSny

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Note M2(1+M1) = M2 + M2M1

How can you rewrite the right hand side using the fact that M1 and M2 commute?
 
Note M2(1+M1) = M2 + M2M1

How can you rewrite the right hand side using the fact that M1 and M2 commute?
##M_2 + M_1M_2##
 

TSny

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OK. Can you see something you can do with this expression that will help?
##(1 +M_1)M_2## ?
 

TSny

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Yes.
 

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