NIMCET 2013 Question Paper with solutions

Question : 15 of 120

Marks: +1, -0

Let Tn denote the number of triangles which can be formed by using the vertices of a regular polygon of nsides. If

T_{n+1}-T_n

= 21 then n equals:

Solution:

As per the given condition,

T_n

triangle can be formed by using the vertices of a regular polygon of n.

∴^{n+1}C_3-^nC_3=21⇒^{7+1}C_3-^7C_3=21

Clearly,

n=7

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