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Homework Help: Find the equation of the form y = Ce^kt if it passes through two specific points

  1. Apr 12, 2010 #1
    1. The problem statement, all variables and given/known data
    Find the equation of the form y = Cekt if it passes through (0, 4) and (5, 1/2)

    2. Relevant equations
    y = mx+b?

    3. The attempt at a solution
    When I see "find the equation", my instant reaction is to find the derivative of y = Cekt. Once I have the derivative (or the slope of the equation I'm finding) I can plug it into y = mx+b. Plug in one set of points I was given to find "b", and plug "b" back into the original equation along with my derivative/slope. However, I don't know what to do with the second pair of points. Do I use it to check my work? Nevertheless, if my reasoning is correct, I'm not sure how to find the derivative of y = Cekt because there are three unknown variables. This is what I have so far:

    y = C(ekt)' + ektC'
    y = C(ekt)(kt)' + 0

    I stopped here because I noticed it didn't look right. If I continue with this derivative, I end up with 0 because the derivative of kt will be 0 (assuming they are both variables) making the first part of the equation zero, and like the first part, the derivative of C is zero (assuming it is a variable) making the second part of the equation zero (thus having 0 + 0).

    Which leads to another question: when do I find the derivative and when do I find the integral? In my other homework problems, I was given the slope and a pair of points. However, this problem was solved by integrating the slope and getting another equation. Then, with the integral, the book plugged in the point and got the particular solution they asked for.
  2. jcsd
  3. Apr 12, 2010 #2


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    I don't understand what you are attempting to do. They gave you y = Cekt and are asking for the equation if it passes through (0, 4) and (5, 1/2).

    They want you to find C and k.
  4. Apr 12, 2010 #3
    Oh.. I must have misinterpreted the problem. So that means I plug in my x coordinates for t and y coordinates for y, right?
  5. Apr 12, 2010 #4


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  6. Apr 12, 2010 #5
    Mmk. Thank you for clarifying the problem for me!
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