in the binomial expansion of (a + b)n , the coefficient of the 4th and the 13th terms are equal to each other. find n?

Coefficient of 4th term would be C3n and coefficient of 13th term would be C12n
Given

nC3=nC12We know that nCr=nCn-rSo,nC3=nCn-12Equating,3=n-12n=15

  • 14

nC3=nC12

it holds only when 3+12=n

thus n=15

  • 4

thanks

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