Question

Let $n\ge 2$ be an integer. Take n distinct points on a circle and join each pair of points by a line segment. Colour the line segment joining every pair of adjacent points by blue and the rest by red. If the number of red and blue line segments are equal, then the value of n is ?___

Answer 1

Number of line segment joining pair of adjacent point = $n$
Number of line segment obtained joining n points on a circle $={}^n{C}_2$
Number of blue line segments = $n$
Number of red line segments = ${}^n{C}_2-n$
${\therefore }^n{C}_2-n=n\Rightarrow \frac{n(n-1)}{2}=2n\Rightarrow n=5$