If we draw diagonals from a fixed vertex of a n sided polygon, there will be (n-2) number of triangles formed. So for n=11, number of triangles formed = 9 for n=12, number of triangles formed = 10 for n=13, number of triangles formed = 11 for n=14, number of triangles formed = 12 for n=15, number of triangles formed = 13
