2019年GRE数学考点：Conditional

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　　Conditional

　　Probability of event A conditioned on the fact that event B has happened is
called the conditional probability and written as P(AIB).

　　P(AIB)=Probability Both A and B happens/ Probability B happens.

　　Example:

　　A six-sided die with faces from 1 to 6 is rolled twice. What is the
probability that the face 5 turned up on first try given that the sum of the two
faces is 9?

　　P(sum of faces is 9)=4/36=1/9.

　　P(face 5 turns up on the first try and sum is 9)=(1/6)*(1/6)=1/36.

　　P(face 5 turned up on first try given sum is 9)=(1/36)*(1/9)=1/4

　　Remember:

　　If P(AIB)=P(A),then two events A and B are independent, that is occurrence
of one does not influence the other