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n = 1 2016 n ( 2016 n ) ( 2016 n 1 ) = ? \Large \displaystyle \sum^{2016}_{n=1}n\dfrac{\binom{2016}{n}}{\binom{2016}{n-1}}= \, ?


This problem was given to me by Anshuman Bais .


The answer is 2033136.

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1 solution

Rohit Udaiwal
Jan 3, 2016

n ( 2016 n ) ( 2016 n 1 ) = n 2016 ! n ! ( 2016 n ) ! 2016 ! ( n 1 ) ! ( 2017 n ) ! = n ( n 1 ) ! ( 2017 n ) ! n ! ( 2016 n ) ! = n ( n 1 ) ! ( 2017 n ) ( 2016 n ) ! n ( n 1 ) ! ( 2016 n ) ! n\cdot\dfrac{\binom{2016}{n}}{\binom{2016}{n-1}}=n\cdot\dfrac{\frac{2016!}{n!(2016-n)!}}{\frac{2016!}{(n-1)!(2017-n)!}}=n\cdot\dfrac{(n-1)!(2017-n)!}{n!(2016-n)!}=n\cdot\dfrac{(n-1)!(2017-n)(2016-n)!}{n(n-1)!(2016-n)!} = 2017 n =2017-n n = 1 2016 n ( 2016 n ) ( 2016 n 1 ) = n = 1 2016 2017 n = 2016 + 2015 + + 1 = 2016 2017 2 = 2033136. \therefore \displaystyle \sum^{2016}_{n=1}n\dfrac{\binom{2016}{n}}{\binom{2016}{n-1}}=\displaystyle \sum^{2016}_{n=1}2017-n \\ =2016+2015+\ldots+1=\dfrac{2016\cdot2017}{2} \\ =2033136.


Note : ( a b ) = a ! b ! ( a b ) ! \large{\binom{\color{#20A900}{a}}{\color{#3D99F6}{b}}=\dfrac{\color{#20A900}{a}!}{\color{#3D99F6}{b}!(\color{#20A900}{a}-\color{#3D99F6}{b})!}}

Nicely done !! (+1)

Akshat Sharda - 5 years, 5 months ago

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