addition law (probability) If A and B are two events then the addition law states that the Pr(A or B) = Pr(A) + Pr(B) − Pr(A and B), or using set notation P(A ∪ B) = P(A) + P(B) − P(A ∩ B). In the special case where A and B are mutually exclusive events this reduces to P(A ∪ B) = P(A) + P(B).
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