# summation algebra question

Printable View

• Oct 8th 2013, 07:13 PM
kingsolomonsgrave
summation algebra question
Attachment 29419

I am not sure what is going on here, when I manipulate the equation I still can't get the required answer. I keep getting

$\frac{{}\sigma^2 \sum({x_i -\bar{x}}) +\sigma^2\sum(x_i)^2}{N\sum({x_i -\bar{x})^2}}$
• Oct 13th 2013, 02:53 PM
votan
Re: summation algebra question
Quote:

Originally Posted by kingsolomonsgrave
Attachment 29419

I am not sure what is going on here, when I manipulate the equation I still can't get the required answer. I keep getting

$\frac{{}\sigma^2 \sum({x_i -\bar{x}}) +\sigma^2\sum(x_i)^2}{N\sum({x_i -\bar{x})^2}}$

Attachment 29459
• Oct 20th 2013, 10:36 AM
kingsolomonsgrave
Re: summation algebra question
well I get that the first term in the numerator of my answer is zero since it's the sum of xi and xbar but what about the denominator? isn't it supposed to stay as is? Is it a typo or should the denominator change from

the sum of (xi minus xbar)^2 to

the sum of (xi minus xbar^2)
• Oct 20th 2013, 11:58 AM
kingsolomonsgrave
Re: summation algebra question
So I've been trying to simplify the following for entirely too long: $\frac{\sigma^2}{n}+\bar{x}^2\frac{\sigma^2}{\sum (x_i -\bar{x}^)^2}$

becomes $\frac{\sigma^2}{n}+\frac{{}\sigma^2\frac{1}{n}\sum x_i^2}{\sum (x_i - \bar{x})^2}$

but WHY did xbar^2 become 1/n times the sum of x_i squared? shouldn't the 1/n be squared there as well? shouldn't $\bar{x}^2$ be equal to $(\frac{1}{n}\sum(x_i))^2$ which would square the $\frac{1}{n}$as well?

Also this is supposed to simplify to

$\frac{\sigma^2 \sum x_i^2}{\sum (x_i -\bar{x}^)^2}$

but I can't get their either. I specifically would like to know this:

If we let sigma^2 =A
N=B
1/n sum of xi squared = C
and sum of (xi-xbar)^2 = D

then for the simplification we have

$\frac{A}{B} + \frac{AC}{D}$ which is supposed to simplify to $\frac{AC}{D}$

except that $\frac{A}{B} + \frac{AC}{D}$ simplifies to

$\frac{A}{B}+\frac{AC}{D} = \frac{AD}{BD}+\frac{ACB}{DB} = \frac{AD+ABC}{DB}=A\frac{D+BC}{DB}$

as far as I can see and that does not compute with the correct answer. which is supposed to be $\frac{AC}{D}$= $\frac{\sigma^2\frac{1}{n}\sum x_i^2}{\sum (x_i-\bar{x})^2}}$

If anyone can just tell me what I'm supposed to do here it would be great.

Or is sigma^2/n = to zero for some reason?

￼￼￼
• Oct 22nd 2013, 02:31 AM
kingsolomonsgrave
Re: summation algebra question
I think I figured it out, the key is that the sum of xi^2 is the same as the sum if xbar^2 Attachment 29539