Can you help me solve this problem?
A regular 13-sided polygon is inscribed in a circle with centre $O$. Triangles can be formed by choosing three vertices of this polygon to be the vertices of the triangle.
How many of the triangles that can be formed in this way have the point $O$ inside?
I have no idea how to approach this question, so I tried to start off simple by drawing the number of triangles that can be inscribed in a square and a pentagon. However, after finding the values 4 and 8, I have no idea where to go from here.
Any help is highly appreciated. Thank you!