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not sure where they got this equation watch

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    i know that xbar is the avg of mail-orders and that ybar is the avg of bookstore.



    however i'm confused as to how they got the total for (xi-xbar)(yi-ybar).

    any help? thanks
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    xi is just x really. its xi in the formula booklet for a level. my teacher said to just take it as x.
    thats the sum of all the numbers take the mean squared.
 
 
 
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Updated: January 15, 2010

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