# Homework Help: Combinatorics Class - Sum Question

1. Sep 17, 2012

### theRukus

1. The problem statement, all variables and given/known data
For any positive integer n determine:

$\sum\limits^n_{i=0} \frac{1}{i!(n-i)!}$

2. Relevant equations

I don't really know where to start.. Up until this point we've just been doing permutations, combinations, and determining the coefficient of a certain term in the expansion of a polynomial. There aren't any examples like this question in the text, and so I am unsure as to what sort of an answer they are looking for... Are they just looking for a general formula (not a sum) for the answer, with n as a variable? Cheers for any direction!!

3. The attempt at a solution

2. Sep 17, 2012

### jbunniii

Hint: does this look familiar?

$$\frac{n!}{i!(n-i)!}$$

3. Sep 17, 2012

### theRukus

So the answer I'm looking for is

$\frac{\dbinom{n}{i}}{n!}$

Correct?

4. Sep 17, 2012

### theRukus

Or will it be

$\sum\limits^n_{i=0} \dfrac{\dbinom{n}{i}}{n!}$

I'm confused as to whether the sum is still involved.

5. Sep 17, 2012

### damabo

you should find the following sum:

$\frac{1}{n!}*\sum \frac{n!}{i! (n-1)!}$

6. Sep 17, 2012

### Ray Vickson

Of course the sum is still involved. The final answer must be in terms of n alone: it cannot contain "i", since all values of i have been summed over. Anyway, just multiplying and dividing by n! does not magically get rid of the sum.

RGV