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Math Help - finding variables.

  1. #1
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    finding variables.

    This could possibly be the second last math question i ever need help with =)


    P=e^-kr

    when...
    P= proportion of cells which survive
    r= radiation level
    k= a constant

    if 40% of cells survive (P), then r=500. when P=1% what would 'r' equal?

    any help please?
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  2. #2
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    0.4=e^{-500k}

    Now solve for k
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  3. #3
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    hmmm k= ln0.4/500
    therefore k= -1.83*10^-3?
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  4. #4
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    0.4=e^{-500k}

    -500k=\ln(0.4)

    k=\frac{-\ln(0.4)}{500}
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  5. #5
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    hmmm ok. so when p=0.01 and k= (-ln0.4)/500r r=2512.9?

    is that what you got?
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  6. #6
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    P=e^{-kr}

    k=\frac{-\ln(0.4)}{500}

    P=e^{-\frac{-\ln(0.4)}{500}r}

    P=e^{\frac{\ln(0.4)}{500}r}

    P=0.01

    0.01=e^{\frac{\ln(0.4)}{500}r}

    \ln(.01)=\frac{\ln(0.4)}{500}r

    \frac{\ln(.01)}{\frac{\ln(0.4)}{500}}=r

    r=\frac{\ln(.01)}{\frac{\ln(0.4)}{500}}

    I get what ever that equals.
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  7. #7
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    thanks. yeah that's the same answer. you're so smart!
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