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Math Help - finding variance of truncus pdf

  1. #1
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    finding variance of truncus pdf

    I know that E(X^2) is the definite integral of x^2 * f(x).

    But what if f(x) = (4/3)(x^-2) ? The answer in the texbook says it is [4x/3].

    I'm so confused. Thanks in advance
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by freswood
    I know that E(X^2) is the definite integral of x^2 * f(x).

    But what if f(x) = (4/3)(x^-2) ? The answer in the texbook says it is [4x/3].

    I'm so confused. Thanks in advance
    There is something wrong with the question. Your f(x) is not a density without
    a statement about its domain, which in this case would probably be
    (4/3, infty). Then as the integration "integrates out" the dependency on x,
    what you report as the textbooks answer cannot be the answer to the
    question you have posted.

    RonL
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