How many diagonals does a quadrilateral have?

How many diagonals does a quadrilateral have?
  1. $1$
  2. $2$
  3. $3$
  4. $4$

Solution

Number of diagonals $= \dfrac{n(n-3)}{2} = \dfrac{4 \cdot 1}{2} = 2$.

Asked in: IMO

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