Addition principle

Counting principle in combinatorics

In combinatorics, the addition principle or rule of sum is a basic counting principle. Stated simply, it is the intuitive idea that if we have A number of ways of doing something and B number of ways of doing another thing and we can not do both at the same time, then there are A + B {\displaystyle A+B} ways to choose one of the actions. In mathematical terms, the addition principle states that, for disjoint sets A and B, we have | A ∪ B | = | A | + | B | {\displaystyle |A\cup B|=|A|+|B|} , provided that the intersection of the sets is without any elements.

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