Two lines intersect at O. Points $A_1,A_2,........A_n$ are taken on one of them and $B_1,B_2,........B_n$ on other. The no. of triangles that can be drawn from with the help of these 2n points and point 'O' is ?

Question

$\begin{array}{1 1} ^{2n+1}C_3 \\ n^3+n^2+2 \\ ^{2n+1}C_3-2.^nC_3+1 \\ n^3 \end{array}$