How many diagonals does a n 18-gon have?

To find the number of diagonals in an 18-gon, you can use the formula ( D = \frac{n(n-3)}{2} ), where ( n ) is the number of sides. For an 18-gon, this gives ( D = \frac{18(18-3)}{2} = \frac{18 \times 15}{2} = 135 ). Therefore, an 18-gon has 135 diagonals.