=+If B1 = A, and By = A , NAfn . . . nAL _, then the B, are disjoint and Ux -, AK = Ux_1 B ., so that P(U" , AK) ="_P(B). Since P(BA) ≤

P(A]) by monotonicity, this establishes the finite subadditivity of P:

P [ U AX|S CP(A).

(2.10)

k=1 k=1