SATHEE: Permutations And Combinations Question 371

Permutations And Combinations Question 371

Question: Let $ T _{n} $ denote the number of triangles which can be formed using the vertices of a regular polygon of n sides. If $ {T _{n+1}}-T _{n}=21 $ , then n equals

Options:

A) 5

B) 7

C) 6

D) 4

Show Answer

Answer:

Correct Answer: B

Solution:

[b] $ ^{n+1}C_3{{-}^{n}}C_3=21 $ $ \because {{}^{n}}C _{r}{{+}^{n}}{C _{r-1}}{{=}^{n+1}}C _{r} $ $ {{\Rightarrow }^{n}}C_2=21\Rightarrow n=7 $

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Permutations And Combinations Question 371

Question: Let $ T _{n} $ denote the number of triangles which can be formed using the vertices of a regular polygon of n sides. If $ {T _{n+1}}-T _{n}=21 $ , then n equals

Options:

Answer:

Solution:

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