Q.28. If Tn denotes the number of traingles which can be formed using the vertices of regular polygon of n sides and Tn+1 – Tn = 21, then n=
A. 5
B. 7
C. 6
D. 4
Answer :- B. 7
Explanation :- According to given condition Tn = nC3
Tn+1 – Tn = 21
n+1C3 – nC3 = 21
Note that n=7 satisfies the above condition
Option (B) is correct.