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Permutations and Combinations question

2014 · Shift 1 · Q23
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Permutations and Combinations question

2014 · Shift 1 · Q23

JEE AdvancedMathematicsPermutations and CombinationsNumerical+3 / −1
Let n≥2{n \ge 2}n≥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
Numerical answer
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Correct answer: 5

  1. Count the total number of line segments formed by joining every pair of points

    With nnn distinct points on a circle, joining every pair gives (n2)=n(n−1)2\binom{n}{2} = \frac{n(n-1)}{2}(2n​)=2n(n−1)​ line segments in total.

  2. Count the blue line segments

    Adjacent points on a circle form the sides of the polygon determined by the nnn points.

    Hence, the number of blue line segments is exactly n.n.n.

  3. Count the red line segments

    All remaining segments are red, so red=(n2)−n.\text{red} = \binom{n}{2} - n.red=(2n​)−n.

  4. Use the condition that red and blue segments are equal

    Given: (n2)−n=n\binom{n}{2} - n = n(2n​)−n=n

    So, (n2)=2n\binom{n}{2} = 2n(2n​)=2n

    Substitute (n2)=n(n−1)2\binom{n}{2} = \frac{n(n-1)}{2}(2n​)=2n(n−1)​: n(n−1)2=2n\frac{n(n-1)}{2} = 2n2n(n−1)​=2n

    Multiply by 222: n(n−1)=4nn(n-1) = 4nn(n−1)=4n

    Since n≥2n \ge 2n≥2, divide by nnn: n−1=4n-1 = 4n−1=4 n=5n = 5n=5

  5. Conclusion

    The required integer value is 5.\boxed{5}.5​.

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