Which of the following describes a concave polygon?

Which of the following describes a concave polygon?
  1. A polygon with all interior angles less than $180^{\circ}$
  2. A polygon with at least one interior angle greater than $180^{\circ}$
  3. A polygon with all sides equal
  4. A polygon with all angles equal

Solution

A concave (non-convex) polygon has at least one reflex interior angle, i.e. an angle greater than $180^{\circ}$.

Asked in: IMO

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