1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Calculate the variance

  1. Oct 26, 2014 #1
    1. The problem statement, all variables and given/known data
    Find the variance of Y=3x^2+3x+3

    3. The attempt at a solution
    Let Y = 3x^2 +3x +3

    Var(Y) = Var(3x^2 +3x +3) = 9Var(x^2) +9Var(x) = 9 [E[X^4] - E[X^2]^2 +E[X^2] - E[X]^2]
    This is wrong.
    Last edited: Oct 26, 2014
  2. jcsd
  3. Oct 26, 2014 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I guess the z should be X, or 9var(x) should be 9Var(Z). Either way, what assumption does that step require?
  4. Oct 26, 2014 #3
    Oh. It assumes no covariance.
    Last edited: Oct 26, 2014
  5. Oct 26, 2014 #4

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    You have assumed that ##\text{Cov}(X^2,X) = 0,## which is not generally true.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted