1.

In the binomial expansion of `(a+b)^n`, the coefficient of the 4th and 13th terms are equal to each other. Find n

Answer» General term in `(a+b)^n,(T_(r+1)) = C(n,r)a^(n-r)b^r `
`:. T_4 = C(n,3)a^(n-3)b^3`
`T_13 = C(n,12)a^(n-12)b^12`
We are given coefficients of 4th and 13th terms are equal to each other.
`:. C(n,3) = C(n,12)`
We know, `C(n,12) = C(n,(n-12))`
`:. C(n,3) = C(n,(n-12))`
`=> 3 = n-12`
`=> n = 15`


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