# Homework Help: What is the average of (Ri-1/N*Sum[Ri])^2 The sum is from i=1 to i=N.

1. Oct 25, 2011

### jaobyccdee

What is the average of (Ri-1/N*Sum[Ri])^2 The sum is from i=1 to i=N.

2. Oct 25, 2011

### HallsofIvy

Re: Summation

What is "Ri"?

3. Oct 25, 2011

### I like Serena

Re: Summation

Hi jaobyccdee!

1/N*Sum[Ri] is the average $\bar R$ of $R_i$.

So you're asking about ${1 \over N}\sum (R_i - \bar R)^2$

Does that look familiar to you?

4. Oct 25, 2011

### jaobyccdee

Re: Summation

er....actually that's what i start with....but i don't know that it should be familiar....

5. Oct 26, 2011

### I like Serena

Re: Summation

The average of the deviation squared is called "variance".
Its square root is called "standard deviation".

6. Oct 26, 2011

### jaobyccdee

Re: Summation

Sorry, i should have mentioned that the question starts with sqrt<R^2>, and asks to prove that it is equal to sqrt(Lζ/3). This is the relationship between contour length and the radius of gyration. So i was trying to evaluate it to something that looks like sqrt(Na/6) where Na^2 is the variance.

7. Oct 26, 2011

### I like Serena

Re: Summation

I can't make chocolate of all these symbols you just introduced here...