If In is the area of $n$ sided regular polygon inscribed in a circle of unit radius and On be the area of the polygon circumscribing the given circle, prove that $$I _ { n } = \frac { O _ { n } } { 2 } \left( 1 + \sqrt { 1 - \left( \frac { 2 l _ { n } } { n } \right) ^ { 2 } } \right)$$