Question

Let a , a , be an arbitrary infinite sequence of events, and let b , b , be 12 12 another infinite sequence of events defined as follows:

Answer 1

So write this and solve your problem.
Pr(Uⁿsubscript i=1 A subscript i) = ∑ⁿ subscript i=1 Pr(Bi) for n=1, 2, 3,.......