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Homework Help: 'changing the subject'

  1. Oct 27, 2008 #1
    1. The problem statement, all variables and given/known data

    Its back to college for me and studying for Civil Engineering Qualification. Im stuck on this one and dont really have anyone to turn to!

    2. Relevant equations

    P1 = P0e-kt

    3. The attempt at a solution

    I need to change the subject to (k) but our lecturer did not cover this type of question!
  2. jcsd
  3. Oct 27, 2008 #2


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    Science Advisor

    If you are going back to college I'm sure you learned long ago that you solve an equation by "undoing" what has been done.
    There have been three things done to k (and you want to solve for k):
    (1) it has been multiplied by -t
    (2) the exponential has been taken
    (3) that has been multiplied by P0

    To undo that, do the opposite in the opposite order.
    (3) To undo "multiply by P0", do the opposite: divide by P0. Dividing both sides of the equation by P0 gives You P/P0= e-kt.
    (2) To undo the exponential use its inverse function, natural logarithm.
    Taking the natural logarithm of both sides give ln(P/P0)= ln(e-kt)= -kt.
    (3) To undo the multiplication by -t, divide by -t:
    I let you both sides by -t.
  4. Oct 27, 2008 #3
    So is the answer...

    k = ln(p1/Po) /-t
    Last edited: Oct 27, 2008
  5. Oct 27, 2008 #4
    Yep that's right!!
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