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I can not understand how to get from one side to the other.

$\sum [(x_i- \bar{x})(y_i - \bar{y})] = \sum[x_i(y_i - \bar{y})]$

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You need to show $\sum \overline{x} (y_i - \overline{y}) = 0$. This sum is equal to $\overline{x} \left(\sum y_i - \sum \overline{y} \right)$ and you have $\sum \overline{y} = n \overline{y}$ since you're just summing up the same number $n$ times. I'll let you try to finish it from here. (Hint: What is the definition of $\overline{y}$ in terms of $y_1,\dots,y_n$?)

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    forgot about the dang + sign at the end... took me a minute. Thanks again for the help.2012-03-29