The number of diagonals in a hexagon is ______.
9
Idea: A diagonal joins two vertices that are not next to each other.
Formula:
\[ ext{Diagonals in an } n ext{-gon} = rac{n(n-3)}{2} \]
For a hexagon:
\[ n = 6 \]
\[ n - 3 = 6 - 3 = 3 \]
\[ n(n-3) = 6 imes 3 = 18 \]
\[ rac{n(n-3)}{2} = rac{18}{2} = 9 \]
So, a hexagon has 9 diagonals.