I need help with a Pre-Calculus question, please help?

There are n vertices in an n-gon. Each vertex has a line to each other vertex except itself and the two neighboring ones. So each vertex has n-3 diagonals, where n is the number of sides. Since there are n vertices, we multiply the answer by n, so n(n -3). This umes that the line between A and B is different from the line between B and A. As we know they are the same, we need count only half of our lines. The other half are the same lines going in the other direction. Thus n(n-3)/2