Summation with binomial coefficients question

  • #1
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Homework Statement


##\sum\limits_{r=0}^n\frac{1}{^nC_r}=a##. Then find the value of $$\sum\sum\limits_{0\le i<j\le n}(\frac{i}{^nC_i}+\frac{j}{^nC_j})$$

Homework Equations



I have used two equations which I derived myself. This is the first one.
20150430_000946-1-1.jpg

The second one is:
20150430_000435-1.jpg


3. The Attempt at a Solution

Using first equation and second equation:
20150430_001236-1.jpg

Now I have to subtract the cases where I=j to get the required sum. But Iis the above conclusion correct? Because I am not getting the required answer after subtracting.
 

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Answers and Replies

  • #2
haruspex
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In the last line of the derivation of your second equation, you have been inconsistent in your substitutions of m for n. Two m's should be m+1.
But I don't understand how you use this equation anyway. a is a function of n, but where you use the equation you seem to be using it as a generic fact for any m, without changing a. I.e. you cannot now substitute n for m as being equal.
 
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  • #3
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Then how do you find ##(2n+1)\sum\limits_{r=0}^n\frac{r}{^nC_r}##?
 
  • #4
haruspex
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Then how do you find ##(2n+1)\sum\limits_{r=0}^n\frac{r}{^nC_r}##?
I haven't solved it myself, yet.
You could try concentrating on one of the two terms in the double sum. You should be able to sum that over the 'other' variable.
 

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