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Prove that the number of subsets of a set containing n distinct elements is 2n, for all n N. [NCERT EXEMPLAR]

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Solution

Let the given statement be defined as Pn: The number of subsets of a set containing n distinct elements=2n, for all nN.Step I: For n=1,LHS= As, the subsets of a set containing only 1 element are: ϕ and the set itself.i.e. the number of subsets of a set containing only 1 element=2RHS=21=2As, LHS=RHSSo, it is true for n=1.Step II: For n=k,Let Pk: The number of subsets of a set containing k distinct elements=2k, be true for some kN.Step III: For n=k+1,Pk+1:Let A=a1, a2, a3, ..., ak, b so that A has k+1 elements.So, the subset of A can be divided into two collections; first contains subsets of A which don't have b in them andthe second contains subsets of A which do have b in them.i.e.First collection: , a1, a1, a2, a1, a2, a3, ..., a1, a2, a3, ..., ak andSecond collection: b, a1, b, a1, a2,, a1, a2, a3, b, ..., a1, a2, a3, ..., ak, bIt can be clearly seen that:The number of subsets of A in first collection=The number of subsets of set with k elements i.e. a1, a2, a3, ..., ak=2k Using step IIAlso, it follows that the second collection must have the same number of the subsets as that of the first =2kSo, the total number of subsets of A=2k+2k=2×2k=2k+1.

Hence, the number of subsets of a set containing n distinct elements is 2n, for all n N.

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