# 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 ${}^{\mathrm{n}}\mathrm{C}_{3}$ and coefficient of 13th term would be ${}^{\mathrm{n}}\mathrm{C}_{12}$
Given

• 17

nC3=nC12

it holds only when 3+12=n

thus n=15

• 3

thanks

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