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Exponential relationships to logarithms and straight line graph?

  1. Jul 22, 2009 #1
    it is suspected that cells in a sample are dividing so that the number of cells present at any one time t (measured in seconds) is growing exponentially according to the relationship y = 64 x 2^2t. it would be hard to check this relationship accurately by plotting measurements of y against t, so in practice one can use logarithms to convert it to a linear line equation. sketch a graph of log(base2)y against t, labelling the point where the graph crosses the axes?
    how to i find the points on x and y axis where the line crosses, i believe the line will be a straight line, how do i do this?

    I REALLY HAVE TRIED WITH THIS QUESTION, IT HAS TAKEN ME THE WHOLE DAY TO TRY AND DO, I HOPE SOMEONE CAN SHOW ME HOW TO DO THIS :)
     
  2. jcsd
  3. Jul 22, 2009 #2

    HallsofIvy

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    What, exactly, did you spend all day doing? Taking the logarithm of both sides of [itex]y= 64(2^{2t})[/itex] gives [itex]ln(y)= log(64)+ 2t log(2)[/itex]

    That is of the form Y= at+ b where Y= log(y), a= 2log(2), and b= log(64). It crosses the Y axis when t= 0, at (0, ln(64)) and the t axis when Y= 0, at t= log(64)/(2log(2)).

    I specifically did not give a base for the logarithm because the above is true for any base. It is particularly simple if you use, not "common" or "natural" logarithm, but the logarithm base 2: [itex]log_2(2)= 1[/itex] and [itex]log_2(64)= log_2(2^6)= 6[/itex] so Letting [itex]Y= log_2(y)[/itex], the equation becomes Y= 2t+ 6 with intercepts at (0, 6) and (3, 0).
     
  4. Jul 22, 2009 #3
    cheers mate, i wasnt doing that, i think i need to spend more time and concentration on logs and try a number of different questions on it, thanks again
     
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