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Find the number of all three elements su...

Find the number of all three elements subsets of the set`{a_1, a_2, a_3, a_n}` which contain `a_3dot`

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The correct Answer is:
`.^(n-1) C_(2)`
The number of three elements subsets containing `a_(3)` is equal to the number of ways of selecting 2 elements out of n-1 elements. So, the required number of subsets is `.^(n-1)C_(2)`.
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