



 
Farzan, Here's the model of a proof. Begin with a triangle. The sum of the interior angles makes 180 degrees. Add another triangle externally to any one side. The sum of the new triangles interior angles is also 180. The sum of the interior angles of the polygon (ignoring internal lines) is 180 + the previous total. The number of external sides is 1 + the previous total. Repeat ad nauseam. Cheers,  


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