1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Find Sol for ODE dy/dx=(x+y+2)^2

  1. Sep 16, 2013 #1

    1s1

    User Avatar

    1. The problem statement, all variables and given/known data
    Find the general solution:
    $$\frac{dy}{dx}=(x+y+3)^{2}$$


    2. Relevant equations



    3. The attempt at a solution
    Methods I have learned: separation of variables, integrating factor for linear equations, exact equations, and substitution. I don't even know where to begin on this one. It has a ##y^{2}## term so it isn't linear; it isn't an exact equation; so there must be a substution. But it doesn't look anything like the substitution problems I've done. I expanded it, but that just seems to make it really long. Any hints or help would be greatly appreciated!
     
  2. jcsd
  3. Sep 16, 2013 #2

    fzero

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Obviously the difficulty is that we have ##x+y## appearing in the power on the RHS, so an obvious choice of substitution would involve this combination. Try ##u=x+y+3##. Since this is linear, ##dy/dx## and ##du/dx## are simply related.
     
  4. Sep 16, 2013 #3

    1s1

    User Avatar

    Thanks! Clearly, I don't understand how substitution works ... yet.

    The only substitution we've learned so far is for Bernoulli's Equation, ##\frac{dy}{dx}+P(x)y=f(x)y^{n}##

    Where the substitution is ##u=y^{1-n}##

    I'll run ##u=x+y+3## through and I should get a linear equation like you said.

    Sorry for the newbie ODE question.
     
  5. Sep 16, 2013 #4

    fzero

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Very often substitution involves trial and error, using experience to suggest promising approaches.

    After the substitution the equation is still not linear (it involves ##u^##), but it is separable, which allows us to easily solve it.
     
  6. Sep 16, 2013 #5

    1s1

    User Avatar

    Making the suggested substitution: ##u=x+y+3## and using:
    $$\int \frac{1}{a^{2}+u^{2}}du=\frac{1}{a}tan^{-1}\frac{u}{a}+C$$
    $$x=tan^{-1}(x+y+3)+C$$

    The choice of substitution in this case seems similar to what you would choose when doing integration by substitution, so hopefully that will be a trend and will make picking the stubstution easier. I suppose practicing with different types of substitutions also develops experience on likely beneficial substitutions. It also seems appropriate to choose ##u## so that ##x## and ##y## are simply related. Thanks again fzero!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Find Sol for ODE dy/dx=(x+y+2)^2
  1. Solve Dy/dx=x+y^2 (Replies: 9)

Loading...