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Math Help - Logirithms-- Need Help!

  1. #1
    Junior Member
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    Post Logirithms-- Need Help!

    Hi there I have a couple questions I could not solve, if some one could help that would be great

    1) Find the half-life if 2000g of mathusiasm decays to 1400g in 80 weeks. Round your answer to nearest week.
    -- The formula i get is this 12log(0.7)/log(1/2)=K and my answer is wrong.

    2)
    Find the half-life if 1000g of tspadium decays to 700g in 3 months. Round your answer to the nearest week.

    3)
    The half-life of luvinmathism is 8 days. Write the half life equation that shows the percent, P of luvinmathism in math 12 students after t days.

    Thanks in advance.
    Last edited by Gurp925; May 2nd 2013 at 08:41 PM.
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  2. #2
    MHF Contributor
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    Re: Logirithms-- Need Help!

    Hey Gurp925.

    If you have a model y(t) = A*e^(kt) where A = 2000g solve for k where 1400 = 2000*e^(80*k) meaning k = [ln(7/10)]/80 which using R gives:

    > log(7/10)/80
    [1] -0.004458437

    Try using the above hint to solve the others.
    Thanks from Gurp925
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