How many diagonals does a hexagon have?

How many diagonals does a hexagon have?
  1. $6$
  2. $9$
  3. $12$
  4. $15$

Solution

Number of diagonals of an $n$-gon $= \dfrac{n(n-3)}{2}$. For $n=6$: $\dfrac{6 \cdot 3}{2} = 9$.

Asked in: IMO

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