1) Let Tn be the number of all possible triangles formed by joining vertices of an n -sided regular polygon. If Tn+1-Tn=10, then the value of n is A) 7 B) 5 C) 10 D) 8 Answer: Option BExplanation:Given , Tn=nC3 Tn+1=n+1C3 ∴ Tn+1−Tn=n+1C3−nC3=10(given) ⇒ nC2+nC3−nC3=10 (∵nCr+nCr+1=n+1Cr+1) ⇒ nC2=10⇒n=5