Some Probability Formulas

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Some useful probability formulas

The Addition Rule

Consider 2 events, A and B. To find the probability that either A or B (or both) will occur, we can use the following formula, called the addition rule:

Addition Rule Formula.JPG

We subtract the probability of both A and B occurring, as to not "double count" them. This can seen better by looking at the following Venn Diagram:

A and B Venn Diagram.JPG