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NCERT Class XI Mathematics - Permutations and Combinations - Solutions

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Question : 24 of 41
Marks: +1, -0
How many chords can be drawn through 21 points on a circle?
Solution:  
A chord is formed by joining two points on a circle.
∴ Required number of chords = 21C2{}^{21}C_{2} = 21!2! 19!\frac{21!}{2!\,19!} = 21×202\frac{21\times 20}{2} = 210
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