1. Let A and B be any two events then – 

Note :-

2. If A and B be any two events in a sample spaces S then the probability of occurrence of at least one of the events A and B is P(A U B) = A(A) + P(B) – P(A∩B).

3. If A , B and C are any three events in a sample spaces S then – 

P(A U B U C) = P(A) + P(B) + P(C) – P(A∩B) – P(B∩C) – P(A∩C) + P(A∩B∩C)

4. General form of Addition of Probability :

5. For events A₁ , A₂ ……………………A_n.