Let $T_n$ be the number of all possible triangles formed by joining vertices of a n−sided regular polygon. If $T_{n+1} − T_{n} = 1$ then the value of n is :

Question

$\begin{array}{1 1} (1) 7 \\(2) 5 \\ (3) 10 \\ (4) 8 \end {array}$