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!

Im stuck

  1. May 18, 2004 #1
    ok
    im trying to solve the following equation using standard aux method:

    d^2y/dx^2 + 3dy/dy +2y = cos x with conditions x(0)=-3 and x'(0) = 3

    my aux eqn is:

    ae^x + be^-2x

    and my yp is;

    a sin kx + b cos kx

    i differentiate this twice and substitute into the original equation, and i get:

    (-a sin x - b cos x)+3(a cos x-b sin x) + 2(a sin x+ b cos x) = cos x.

    Now at this stage im not sure if this is right or not, can someone please confirm if this is right or not.

    I ve done this question using the laplace method and the answer i get is;

    -19/2e^x +32/5 e^2x + 1/10 cos x - 3/10 sin x.

    Here my aux equation is not the same as what i get when i apply the laplace method.

    Im not sure what is going wrong.
    Someone help me please.
     
  2. jcsd
  3. May 18, 2004 #2

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    you have a and b representing two different numbers simlutaneously.

    if you're not sure which of e^2x or e^-2x is correct why don't you subs it into the equation to see which one works? i think it is -2, i also think you want e^-x and not e^x since the aux equation is m^2+3m+2=(m+1)(m+2)
     
  4. May 18, 2004 #3
    solution

    Hi;
    Here is your solution:
    [tex]
    y(x)=\frac{3sinx}{10}+\frac{cosx}{10}-\frac{7e^{-x}}{2}+\frac{2e^{-2x}}{5}
    [/tex]
    Best of luck,
    Max.
    P.S. Obtained by Maxima: http://maxima.sourceforge.net/download.shtml
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Im stuck
  1. Simple and stuck (Replies: 1)

  2. Stuck on an Integral? (Replies: 1)

Loading...