In the expansion of (x+y)^n, the coefficient of 4th and 13th term are

1.	In the expansion of (x+y)^n, the coefficient of 4th and 13th term are equal. Find n
Answer» In the expansion of (x + y)n,T4 = T3+1 = nC3xn-3\xa0{tex}\\times{/tex}\xa0y3and T13 = T12+1 = nC12xn-12\xa0{tex}\\times{/tex}\xa0y12We are given that coefficients of 4th and 13th terms are equal.{tex}\\therefore{/tex}\xa0nC3 = nC12{tex}\\Rightarrow{/tex}\xa0nC3 = nCn-12 [{tex}\\because{/tex}\xa0nCr = nCn-r]{tex}\\therefore{/tex}\xa0n - 12 = 3\xa0{tex}\\Rightarrow{/tex}\xa0n = 15

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