The first term, f(1) = 4, to find the second term,
which is f(2-1) = f(1) = 4, so second term, f(2) = 4+5
hence 9, to find the terd term,
which is f(3-1) = f(2) = 9, so terd term, f(3) = 9+5
hence 14, so common difference is 5.
12 term is 59.
The event that either M1 or M2 fails has probability
by the addition rule. Failure events are independent, so
so that
Denote this probability by . Then follows a geometric distribution with this parameter and has density
The expectation is .