JEE Challenger
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Equal Number of Red and Blue Segments on Circle

Consider n≥2n \ge 2 distinct points positioned on a circle, with line segments joining every pair of points. Line segments connecting adjacent points are colored blue, and the remaining ones are colored red. If the total number of red segments equals the number of blue segments, find the value of nn.

Official Numerical Answer5

Step-by-Step Solution

To find the value of nn, we analyze the total number of line segments that can be formed by joining nn distinct points on a circle.

  1. Total Number of Line Segments: The total number of line segments formed by connecting any two of the nn distinct points is given by the combination formula: Total segments=(n2)=n(n−1)2\text{Total segments} = \binom{n}{2} = \frac{n(n-1)}{2}

  2. Number of Blue Segments: The line segments connecting adjacent points along the circle form an nn-sided polygon. Therefore, the number of blue segments is equal to nn: Blue segments=n\text{Blue segments} = n

  3. Number of Red Segments: The remaining line segments (which represent the diagonals of the nn-gon) are colored red. Thus, the number of red segments is: Red segments=(n2)−n=n(n−1)2−n\text{Red segments} = \binom{n}{2} - n = \frac{n(n-1)}{2} - n

  4. Equating Red and Blue Segments: We are given that the number of red segments equals the number of blue segments: n(n−1)2−n=n\frac{n(n-1)}{2} - n = n

    Adding nn to both sides gives: n(n−1)2=2n\frac{n(n-1)}{2} = 2n

    Since n≥2n \ge 2, we have n≠0n \neq 0. Dividing both sides of the equation by nn: n−12=2\frac{n-1}{2} = 2

    Multiplying both sides by 22: n−1=4n - 1 = 4 n=5n = 5

Thus, the required value of nn is 55.