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Math Help - interpreting formulas properly

  1. #1
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    interpreting formulas properly

    All my life i seem to interpret formulas incorrect. ive never really tried in school until now. ive been in college for awhile. i know im pathedic. Anyways, i have a relatively simple question. Forgive me because i dont know how to superscribe and XXXX i cant think of the terms. anyways. im donig hte least squares regression line.
    E=sum in my equation.

    S_x_y=E X_i Y_i

    why do i multiply X*Y individually and then sum that entire row?

    Why dont i multiply the sum of the X's times the sum of the Y's?


    If anyone knows somewhere that i could work on interpretting formulas id appreciate it because it seems that it can be fixed rather quickly while it causes lots of grief.

    ___________

    Also, why sometimes does it show Yhat sub i = bsub0 +bsub1*Xsubi

    whats the difference between sub i and sub 1? thanks
    Last edited by brett701; September 11th 2010 at 10:16 PM.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by brett701 View Post
    All my life i seem to interpret formulas incorrect. ive never really tried in school until now. ive been in college for awhile. i know im pathedic. Anyways, i have a relatively simple question. Forgive me because i dont know how to superscribe and XXXX i cant think of the terms. anyways. im donig hte least squares regression line.
    E=sum in my equation.

    S_x_y=E X_i Y_i

    why do i multiply X*Y individually and then sum that entire row?

    Why dont i multiply the sum of the X's times the sum of the Y's?
    Because that is what the notation means, summing the product is different form multiplying the sums and:

    \displaystyle \sum_{i=1}^n x_iy_i

    is defined to mean:

    x_1y_1+x_2y_2+... + x_ny_n

    CB
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