- #1
Saladsamurai
- 3,020
- 7
Homework Statement
I have this nasty summation and I am close to finding a way to calculate it with my graphing calculator. I just need to iron out the details. If I can rewrite the summation on terms of [itex]\bar{x}[/itex], [itex]\bar{y}[/itex] and [itex]\sum x_iy_i[/itex] I will be all set. I will explain these terms in a moment. First, here is the sum:
[tex]S_{xy} = \sum_{i=1}^n (x_i - \bar{x})(y_i - \bar{y})[/tex]
Where [itex]x_i[/itex] and [itex]y_i[/itex] are the x and y (sample) averages.
If I do out the multiplication I get [tex]S_{xy} =
\sum_{i=1}^n (x_i - \bar{x})(y_i - \bar{y})[/tex]
[tex] = \sum x_iy_i - x_i\bar{y} - y_i\bar{x} + \bar{x}\bar{y}[/tex]
[tex] = \sum x_iy_i - \bar{y}\sum x_i - \bar{x}\sum y_i +\sum\bar{x}\bar{y}[/tex]
Now I it is just the last term [itex]\sum\bar{x}\bar{y}[/itex] that is bothering me. I think the answer is obvious, but I would like confirmation: is the expression [itex]\sum\bar{x}\bar{y}[/itex] identical to [itex]n*(\bar{x}\bar{y})[/itex] ?It must be. I don't know why I am doubting this.