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Math Help - Poisson

  1. #1
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    Poisson

    \lambda=4

    \displaystyle P(X\geq 5|X\geq 2)=\frac{P(X\geq 5\cap X\geq 2)}{P(X\geq 2)}=\frac{P(X\geq 5)}{P(X\geq 2)}=\frac{1-P(X<5)}{1-P(X<2)}

    When I did this, the answer was incorrect. Is this process wrong and why?
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  2. #2
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    Looks correct to me. Was your answer even close?

    I get \displaystyle \frac{1-\left(\frac{4^4e^{-4}}{4!}+\frac{4^3e^{-4}}{3!}+\dots +\frac{4^0e^{-4}}{0!}\right)}{1-\left(\frac{4^1e^{-4}}{1!} +\frac{4^0e^{-4}}{0!}}\right)
    Last edited by pickslides; March 17th 2011 at 04:58 PM. Reason: bad latex.
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  3. #3
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    Quote Originally Posted by pickslides View Post
    Looks correct to me. Was your answer even close?

    I get \displaystyle \frac{1-\left(\frac{4^4}{4!}+\frac{4^3}{3!}+\dots +\frac{4^0}{0!}\right)}{1-\left(\frac{4^2}{2!}+\frac{4^1}{1!} +\frac{4^0}{0!}}\right)
    You forgot e^{-4}, but I obtain .4085 and the answer is .37 somethingish.
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  4. #4
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    Quote Originally Posted by dwsmith View Post
    You forgot e^{-4},
    I did forget, but stick by my answer (after the edit!). Sorry dw, might have to wait for another forum member to come through and eyeball the problem.

    Maybe the book's answer is wrong?
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    Quote Originally Posted by dwsmith View Post
    \lambda=4

    \displaystyle P(X\geq 5|X\geq 2)=\frac{P(X\geq 5\cap X\geq 2)}{P(X\geq 2)}=\frac{P(X\geq 5)}{P(X\geq 2)}=\frac{1-P(X<5)}{1-P(X<2)}

    When I did this, the answer was incorrect. Is this process wrong and why?
    Looks right to me.

    I get 0.4086 as the answer.
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