Binomial coefficients in the denominator!

If the value of the sum

r = 0 1006 2012 2 r ( r + 1 ) ( 2013 r + 1 ) \displaystyle\sum_{r=0}^{1006} \frac{2012-2r}{(r+1)\dbinom{2013}{r+1}}

is S S then find the value of S + 1 ( 2013 1006 ) S+\frac{\displaystyle1}{\dbinom{2013}{1006}} .


The answer is 1.00.

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1 solution

Karthik Kannan
Apr 4, 2014

We have

S = r = 0 1006 2012 2 r ( r + 1 ) ( 2013 r + 1 ) S=\displaystyle\sum_{r=0}^{1006} \frac{2012-2r}{(r+1)\dbinom{2013}{r+1}}

S = r = 0 1006 ( 2013 r ) ( r + 1 ) ( r + 1 ) ( 2013 r + 1 ) S=\displaystyle\sum_{r=0}^{1006} \frac{(2013-r)-(r+1)}{(r+1)\dbinom{2013}{r+1}}

S = r = 0 1006 2013 r ( r + 1 ) ( 2013 r + 1 ) 1 ( 2013 r + 1 ) S=\displaystyle\sum_{r=0}^{1006} \frac{2013-r}{(r+1)\dbinom{2013}{r+1}}-\frac{\displaystyle 1}{\dbinom{2013}{r+1}}

S = r = 0 1006 1 ( 2013 r ) 1 ( 2013 r + 1 ) S=\displaystyle\sum_{r=0}^{1006} \frac{\displaystyle 1}{\dbinom{2013}{r}}-\frac{\displaystyle 1}{\dbinom{2013}{r+1}}

This is a telescopic series.Thus

S = 1 1 ( 2013 1007 ) S=1-\frac{\displaystyle 1}{\dbinom{2013}{1007}}

Thus the required answer is 1 1 ( 2013 1007 ) + 1 ( 2013 1006 ) = 1 1-\frac{\displaystyle 1}{\dbinom{2013}{1007}}+\frac{\displaystyle 1}{\dbinom{2013}{1006}}=1

How does s become one?

Nic Smith - 2 years, 8 months ago

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