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$F$ is a finite measure on $(X,A)$

$a$ and $b$ belong to $A$

show that $F(a \cup b)=F(a)+F(b)-F(a \cap b)$

I have no idea how to approach this question. Any assistance would be appreciated.

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    Venn diagrams? Honestly, I don't know enough about measure theory to give a proper answer, but that looks like the definition of union in the context of measure, so I imagine one could approach it as one approaches normal unions. Then again, I could be way off base.2011-06-19

2 Answers 2