# Without using $m^*(\cup_{i=1}^\infty A_i)=\lim_{n\to\infty}m^*{(A_1\cup…\cup A_n)}$ how to prove it?

Without using $$m^*(\bigcup_{i=1}^\infty A_i)=\lim_{n\to\infty}m^*{(A_1\cup...\cup A_n)}$$ how to prove it?

$m^*$ is the Lebesgue outer measure.

It would follow from the definition of measurable set that $$m^*(A \cup B)= m^*(A)+m^*(B)$$ for every disjoint pair of subsets $A, B \subset \mathbb{R}$ implies that every subset of $\mathbb{R}$ is measurable. However, this is a contradiction, since there is at least one measurable set, eg the Vitali set on $[0,1].$
For the second part, use induction. Certainly the case is true if $n=1$. Now assume it is true for the $(n-1)th$ case, then noting that $(\bigcup_{k=1}^nA_k)\cap A_n=A_n$ and $(\bigcup_{k=1}^nA_k)\cap {A_n}^{C}=\cup_{k=1}^{n-1}A_k$ are disjoint, $$m^*(\cup_{k=1}^{n} A_k)=m^*(A_n)+m^*\left(\cup_{k=1}^{n-1}(A_k)\right)=m^*(A_n)+\sum_{k=1}^{n-1}m^*(A_k)=\sum_{k=1}^{n}m^*(A_k).$$