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Advanced Functions test question

  1. Oct 25, 2007 #1
    A bacteria doubles every 15h find when it will be at 1/16th of its present amount.
    not sure if there were more stuff stated but that's basically the jist of it.

    i'm assuming y=b(2)^(x/15) is the doubling equation


    not so sure how to solve it was going to just put in times 1/16 for the exponent but then i noticed that it was too little steps and the question was out of 4

    (it was on a advanced functions test i just had today lol)
     
  2. jcsd
  3. Oct 25, 2007 #2

    Dick

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    Ok, so let y(x)=b*2^(x/15). If the present time is x=0, you want to solve y(x)=y(0)/16. Put your form into that and solve for x.
     
  4. Oct 30, 2007 #3
    yeah i did that at first (i think it's what your talking about) and it was like

    y(0)/16=2^(x/15)

    2^(0)/16=2^(x/15)

    1/16=2^(x/15)

    2^(-4)=2^(x/15)
    -4=x/15
    -60=x

    and i lost a mark cause the negative (i think...) another student instead subbed 1/16 for b and ended up with +60 and got full marks, just wondering if anyone knows why???
    i only just looked at the question now.
    sucks cause i think that mine was just from different reference...
     
  5. Oct 30, 2007 #4

    Dick

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    y(0)=b. So your first equation should read b/16=b*2^(x/15) and the b cancels. Other than that I'd call the solution fine. If the population is doubling every 15h then the time when it was 1/16 of it's current size was 60 hrs in the past.
     
  6. Nov 1, 2007 #5
    ooooooooohh maybe i lost a mark for not fully solving for y(0) before subbing ... forgot completely about the b tbh.

    thanks man :D
     
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