Problem 1329 (difficulty: 7/10)

Prove that if \(\displaystyle B_1,B_2,\ldots\subset\R^p\) are pairwise disjoint open balls then

\(\displaystyle b\bigg(\bigcup_{i=1}^\infty B_i\bigg) = \sum_{i=1}^\infty b(B_i). \)

Give me another random problem!