Question

Consider the five points comprising of the vertices of a square and the intersection point of its diagonals. How many triangles can be formed using these points?

Answer 1

To form a triangle, $3$ points out of $5$ can be chosen in ${}^5{C}_3=10$ ways.
But of these, the three points lying on the $2$ diagonals will be collinear.
So $10-2=8$ triangles can be formed.