In a regular polygon, the difference between the interior and exterior angle at any vertex is $108^{\circ}$.…

In a regular polygon, the difference between the interior and exterior angle at any vertex is $108^{\circ}$. The number of sides is:
  1. $8$
  2. $10$
  3. $12$
  4. $15$

Solution

Let exterior $= x$, interior $= 180 - x$. Then $(180-x) - x = 108 \Rightarrow 180 - 2x = 108 \Rightarrow x = 36^{\circ}$. So $n = \dfrac{360}{36} = 10$.

Asked in: IMO

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