Given 21 points on a circle

We know that we require two points on the circle to draw a chord

Number of chords is are

^{21}C_{2}=

\( \dfrac{21!}{2!(21-2)!}=\dfrac{21\times 20\times 19!}{2!\times 19!}\)

= \( \dfrac{21\times 20}{2\times 1}\)

= \( \dfrac{420}{2}=210\)

Total number of chords can be drawn are 210

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