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Question

In Tn denotes the number of triangles which can be formed using the vertices of a regular polygon of ‘n’ sides.If Tn+1Tn=21 then n equal
  1. 5
  2. 7
  3. 6
  4. 4


Solution

The correct option is B 7
n+1C3nc3=21nC2=21
n(n1)2=21n=7

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