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Quick math question

  1. Mar 27, 2008 #1
    I am currently working on differential equation. I have integrated both sides and I get:
    p/p-100= C2e^(kt)
    p= (p-100) * (C2e^(kt))

    How do I get p by itself on the left hand side. I need to solve for p.

    Thank you.

  2. jcsd
  3. Mar 27, 2008 #2
    You should show the original equation.
  4. Mar 27, 2008 #3

    dp/dt = kp(1-p/100)

    But all I need to know is how to solve for p in the equation p= (p-100)(C2e^kt).

    Thank you.
  5. Mar 27, 2008 #4


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    I guess it would be naughty of me to say that you shouldn't be doing differential equations if you can't do algebra! (So I won't.)

    Starting from p= (p-100) * (C2ekt), multiply out the right side: p= C2ektp- 100C2ekt. Subtract C2ektp from both sides:p- C2ektp= -100C2ekt so p(1- C2ekt)= -100C2ekt. Finally, divide both sides by 1- C2ekt:
    p= -100C2ekt/(1- C2ekt).
  6. Mar 27, 2008 #5
    Neither do i understand this. People tend to have elementary problems in differentiating and integrating some really simple looking functions, and they do Diff. Equations.
  7. Mar 27, 2008 #6
    I got exactly what you got. I have a very high IQ and am a member of several high IQ groups and I even have a 4.0 at tech. However, the -100C2e^kt/(1- C2e^kt) has to be simplified so I can plug information in for C2 and k and I do not know how to do so. When I ask about anything it means that I have tried everything I know and cannot figure it out or am unsure of my answer. A good teacher and person does not belittle or demean someone for asking a question. SHAME ON YOU!!!
  8. Mar 27, 2008 #7
    Well, i do not know who you are reffering to. But, honestly, it doesn't make any difference at all.BTW, if you are reffering to me, i am not a teacher and neither do i tend to be a good person....lol...
    Nobody here was trying to belittle, offend, blame,demean, contempt, you, or anything like this. And yes, i agree that every question has its value, but i do now withdraw what i said before: It is ridiculous that people have simple algebra problems while they work on Calculus II, Diff. eq, Lin. Algebra or anything like that. Making your way to those courses, somehow means that you already know what preceeds these subjects. I am not saying these things to you personally, but rather in general, so there is no room to get offended.

    If you really want to offend people like this, i would suggest find some other place!
  9. Mar 27, 2008 #8
    Well, if you want to find C2, and k, there have to be some extra information on your original problem.
  10. Mar 27, 2008 #9
    there was: in there year 1990 the pop was 5.28billion and in 2000 the pop was 6.07billion
  11. Mar 27, 2008 #10
    Well, now make use of this fact, that is let t=0 correspond to the year 1990, and t=10, correspond to the year 2000. Now assuming that p (on your equation) is the population you have two initial conditions,
    p(0)=5.28billion and --> from this one you will be able to find C2
    p(10)=6.07billion --> from this one, and also now using the result for C2, you will be able to find k.

    From here on, only some simple algebraic steps are included.
    I think you'll be fine.
    Last edited: Mar 27, 2008
  12. Mar 27, 2008 #11
    Solving the original equation for p was also simple in that sense :smile:
  13. Mar 27, 2008 #12
    Are you suggesting that the op won't be agle to handle this either?

    And i do agree with what u said...lol...
  14. Mar 27, 2008 #13


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    As far as I can see, essentially this same equation is being discussed in another thread.
    'Logistic growth, population, limits'.
  15. Mar 27, 2008 #14
    What link?
  16. Mar 27, 2008 #15


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    That's the title and right now it's 9 posts down from here in this forum stupidmath.:smile:
  17. Mar 27, 2008 #16
    I don't think it is the exact same question though. And defenitely it is asked by someone else.
  18. Mar 27, 2008 #17


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    Homework Helper
    Gold Member

    Not same question but same equation just with different numerical constants.
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