How many interior diagonals does a regular nine-sided polygon have?  (An $\emph{interior diagonal }$ is a segment connecting two vertices which do not have an edge between them.)
To count the interior diagonals of a polygon, I need to know how many vertices it has and how many edges it has. A regular nine-sided polygon has nine vertices and nine edges, so I can use a formula that relates these quantities to the number of diagonals. I recall that for any convex polygon with n vertices, the number of interior diagonals is n(n-3)/2. This formula makes sense, because for each vertex, I can draw n-3 diagonals to the other vertices that are not adjacent to it, and then I divide by 2 to avoid counting each diagonal twice. Therefore, for a regular nine-sided polygon, the number of interior diagonals is 9(9-3)/2 = 9(6)/2 = 27. 