20. 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: (A) 5 (B) 7 (C) 6 (D) 4
20. 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:\\
(A) 5\\
(B) 7\\
(C) 6\\
(D) 4\\