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Homework Help: Construct ODE that approaches an asymptote

  1. Sep 13, 2013 #1
    1. The problem statement, all variables and given/known data
    Construct a first order linear differential equation whose solutions have the required behavior as t approaches infinity. Then solve your equation and confirm that the solutions do indeed have the specified property.

    All solutions are asymptotic to the line y = 2 - t as t approaches infinity.

    I don't even know where to begin, could someone give me a kick in the right direction? I tried just writing a linear differential equations with two unknown functions f and g (y' + fy = g) and getting the general solution but obviously that involves a completely unknown integral and I doubt it would lead anywhere.
  2. jcsd
  3. Sep 13, 2013 #2


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    Let [itex]x(t) = y(t) - (2-t)[/itex].

    Can you think of a linear first-order ODE with constant coefficients for which all solutions tend to zero as [itex]t \to \infty[/itex]?

    Now let [itex]x[/itex] satisfy that ODE.
  4. Sep 15, 2013 #3
    Thanks, could use a few more pointers though. So I put x(t) = y(t) - (2-t) where x approaches zero as t approaches infinity which makes sense as the equation describes what y(t) I want.
    Can't see how I relate it to the ODE though. Say I find one where all solutions tend to zero (y' + y = 0) for example, how do I connect it to x(t)?
  5. Sep 15, 2013 #4


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    Take [itex]x' + x = 0[/itex] and substitute [itex]x = y - (2-t)[/itex].
  6. Sep 15, 2013 #5
    Understand now, thank you. No idea why it was so tricky to wrap my head around it now, but there you go.
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