Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Some simple Differential Equation questions [Please help!]

  1. Feb 1, 2009 #1

    I am taking a course in differential equations.

    So far we have learned the following topics/methods:

    - Separable equations
    - Direction fields
    - Exact equations
    - Linear ODE's - Homogeneous & Non-Homogeneous equations

    1. How do you know if an equation is linear or non-linear. What is the quickest way to tell?
    >> In order to be linear, does it have to have the form: y' + p(x)y = r(x) (Known as the Standard Form). Where p(x) and r(x) are functions of x only or constants?

    2. When y' + p(x)y = 0; where r(x) = 0. Is this a Homogeneous equation? Why?

    3. When y' + p(x)y = r(x); where r(x) is a function of x only or a constant. Is this a Non-Homogeneous equation? Why?

    4. Finally, can someone explain a methodical way to draw direction fields for a given ODE. It has something to do with isoclines. When you draw the direction fields you get tangents (slope lines) which define the direction of the solution curves y(x). Can someone give me an idea how to do this with an example.

  2. jcsd
  3. Feb 3, 2009 #2
    1. More or less....it has to have the format:
    A(x)d^n +An-1(x)d^n-1......=F(x)
    They'll probably only make you know this enough to put an equation into Standard Form that may not initially "appear" to be in Standard Form.

    2. A homogeneous equation will be in the form of y'=f(x,y) where the right side can be expressed as a function of the ratio y/x alone.....one way that usually works as a check is to see if the sum of the powers are the same for each term.
    So: x^3 + x^2y = f(x,y) Both terms add up to degree 3 (3 and 2+1=3)

    I've also seen its definition written like; A homogenous equation is an equation that can be rewritten into the form having zero on one side of the equal sign and a homogeneous function of all the variables on the other side.

    I shy away from any official "definition" for a mathematical term.....if you give an absolute for something involving math....a math guy will show you where you're wrong. ..lol

    3. r(x) is a function of x only.

    4. There should be some examples in your book. My diff eq professor spent about 5 minutes with direction fields...said they were nice to know...and then told us he wouldn't be testing us on them. So, fearful that I may learn something I don't need to know, I never paid attention. lol

    I believe MIT's opencourseware for differential equations with Arthur Mattuck shows field lines in some detail the first lecture. You can find them on youtube.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook