P(A + B) = P(A) + P(B) - P(AB)？

2. Subtraction formula

P(A - B) = P(A) - P(AB)

3. Conditional Probability and Multiplication Formula

P(B/A) = P(AB)/P(A) is the conditional probability of event b under the condition of event a.

Multiplication formula: P(AB) = P(A)P(B/A)

More general: p (a1a2 ... an) = p (a1) p (a2/a1) p (a3/a1a2) ...

4. Total probability formula

Let the events B 1, B2, ..., Bn satisfy:

1.B2 b 1, ..., Bn are mutually incompatible, P (Bi)>0.

2.a belongs to B2 event alliance B 1, ..., Bn.

Then there is a full probability formula: P(A) = P(B 1)P(A/B 1)? +P(B2)P (A/B2) +...+ P (UK) P (USA/UK)