In a polygon with 'n' vertex, we can trace diagonals from one vertex to all vertices, except to the vertex chosen and the two adjacent vertices (because we would have sides and not diagonals), so we would have 'n - 3' diagonals.
If we have a polygon with 15 vertex, the number of diagonals from one vertex is 15 - 3 = 12.
since it is exactly in the halfway between 