Excision Property in Measure Theory

Excision property of measurable sets (Proof)

If A is a measurable set of finite outer measure that is contained in B, then \displaystyle m^*(B\setminus A)=m^*(B)-m^*(A).

Proof:

By the measurability of A,
\begin{aligned}  m^*(B)&=m^*(B\cap A)+m^*(B\cap A^c)\\  &=m^*(A)+m^*(B\setminus A).  \end{aligned}
Since m^*(A)<\infty, we have the result.

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