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Math Help - mean and standard deviation....

  1. #1
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    mean and standard deviation....

    hi all,
    i have a problem with this question....

    A summary of 24 observations of x gave the following information:
    \sum(x-a)=-73.2 and \sum(x-a)^2=2115
    the mean of these value of x is 8.95
    a) find the value of the constant a.
    b) find the standard deviation of the value of x.

    in this following case, that
    \sum(x-a)=\sum (x)-\sum (a) true??

    thanks....
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  2. #2
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    Quote Originally Posted by pencil09 View Post
    hi all,
    i have a problem with this question....

    A summary of 24 observations of x gave the following information:
    \sum(x-a)=-73.2 and \sum(x-a)^2=2115
    the mean of these value of x is 8.95
    a) find the value of the constant a.
    b) find the standard deviation of the value of x.

    in this following case, that
    \sum(x-a)=\sum (x)-\sum (a) true??

    thanks....
    \sum(x-a)=\sum (x)-\sum (a) true?? duno if that's true or not but here is how you do it:

    let y=x-a

    Then \sum y = -73.2 \Rightarrow \overline{y}=\frac{\sum y}{n}=\frac{-73.2}{24}=-3.05

    Now x=y+a \Rightarrow \overline{x} = \overline{y} + a = 8.95 rearrange to find a. where  \overline{x} = 8.95 and \overline{y}=-3.05


    b) \sum y^2=2115 \Rightarrow sdy= \sqrt{\frac{\sum y^2}{n} - \overline{y^2}} = \sqrt{\frac{2115}{24} - 3.05^2} = 8.87 (3s.f.)

    sdx=1 * sdy = 8.87 (3s.f.)
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  3. #3
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    Quote Originally Posted by BabyMilo View Post
    \sum(x-a)=\sum (x)-\sum (a) true?? duno if that's true or not but here is how you do it:

    let y=x-a

    Then \sum y = -73.2 \Rightarrow \overline{y}=\frac{\sum y}{n}=\frac{-73.2}{24}=-3.05

    Now x=y+a \Rightarrow \overline{x} = \overline{y} + a = 8.95 rearrange to find a. where  \overline{x} = 8.95 and \overline{y}=-3.05


    b) \sum y^2=2115 \Rightarrow sdy= \sqrt{\frac{\sum y^2}{n} - \overline{y^2}} = \sqrt{\frac{2115}{24} - 3.05^2} = 8.87 (3s.f.)

    sdx=1 * sdy = 8.87 (3s.f.)
    thanks for the reply...
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  4. #4
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    Quote Originally Posted by pencil09 View Post
    hi all,
    i have a problem with this question....

    A summary of 24 observations of x gave the following information:
    \sum(x-a)=-73.2 and \sum(x-a)^2=2115
    the mean of these value of x is 8.95
    a) find the value of the constant a.
    b) find the standard deviation of the value of x.

    in this following case, that
    \sum(x-a)=\sum (x)-\sum (a) true??

    thanks....
    Asked here: http://www.mathhelpforum.com/math-he...-question.html

    Thread closed.
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