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Homework Help: Variance math help

  1. Apr 17, 2013 #1
    The random variable X1, X2,.....,X15 are independent and take each values ​​ +1,-1 with probability 1/2.

    We define Y = sum from j=1 to 15(j*Xj)

    whats is the variance VAR(Y)=?

    i will find this with VAR(Y)=E(X^2)-(E(X)^2) but how i can find them?
  2. jcsd
  3. Apr 17, 2013 #2


    Staff: Mentor

    Your book probably has some theorems that cover finding the variance of a sum of multiple of random variables, such as the following.

    If X and Y are random variables, and a and b are constants, then
    1. Var(X + Y) = Var(X) + Var(Y) + 2Covar(X, Y)
    2. Var(aX) = a2Var(X)
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