We know that
Ei  and Ej are disjoint, for i
≠j
, therefore, A  ∩ Ei and A  ∩ Ej  are also disjoint for all
i ≠ j,  i, j = 1, 2, , n Thus,
P(A) = P [(A ∩ E1) ∪ (A ∩ E2)∪ ∪ (A ∩ En)]
= P (A ∩ E1) + P (A ∩ E2) +