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Math Help - Word Problem w/exponents

  1. #1
    Newbie crazydaizy78's Avatar
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    Word Problem w/exponents

    The antibiotic clarithromycin is eliminated from the body according to the formula:
    A(t)=500e^-0.1386t
    A is amount remaining of mg remaining in body after (t) hours the drug reaches peak concentration. How much time will pass before the amoun of drug in the body is reduced to 100mg?

    This is as far as I've gotten....I don't think I'm even close.
    100(t)=500e^(-0.1386(t))
    -100 -100
    __________________
    t=400e^(0.1386(t))
    Thanks for any assistance
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  2. #2
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    Hello, crazydaizy78!

    The antibiotic clarithromycin is eliminated from the body according to the formula:
    . . A(t)\:=\:500e^{-0.1386t}

    A is amount remaining in mg remaining in body after (t) hours.
    How much time will pass before the amount of drug in the body is reduced to 100 mg?
    You started correctly . . .

    We have: . 100 \:=\:500e^{-0.1386t}

    Divide by 100: . 1 \;=\;5e^{0.1386t}

    Multiply by e^{0.1386t}\!:\;\;e^{0.1386t} \;=\;5

    Take logs: . \ln\left(e^{0.1386t}\right) \;=\;\ln(5)

    And we have: . 0.1386t\ln(e) \;=\;\ln(5)

    Since \ln(e) = 1, we have: . 0.1386t \;=\;\ln(5)\quad\Rightarrow\quad t \;=\;\frac{\ln(5)}{0.1386}


    . . Therefore: .  t \:\approx\:11.6 hours.

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