If T_n denotes the number of triangles which can be formed using the vertices of regular polygon of n sides and T _ n +1 - T _ n =21 , then n =

Step-by-step solution

According to the given condition, T _ n = ^ n C ₃ T _ n +1 - T _ n =21 ^ n +1 C ₃- ^ n C ₃=21 Note that n =7 satisfies the above condition. Option (B) is correct.