The derivation method of "a set with n elements has two subsets of power n"

Multiplication principle: suppose a subset, a 1 is in the subset or not, and there are two choices; A2 is also two kinds ............................................................................................................................................................................ So the number of subsets is 2 n.

Proper subset excluding the set itself is (2 n)- 1. The nonempty proper subset minus the empty set is (2 n)-2.

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