The interior and exterior angles at any vertex of a polygon are:

The interior and exterior angles at any vertex of a polygon are:
  1. Equal
  2. Complementary (sum to $90^{\circ}$)
  3. Supplementary (sum to $180^{\circ}$)
  4. Always sum to $360^{\circ}$

Solution

Interior and exterior angles at the same vertex form a linear pair, so they sum to $180^{\circ}$.

Asked in: IMO

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