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

Homework Help: Difference equation, ypn guess problem

  1. Jan 1, 2012 #1
    1. The problem statement, all variables and given/known data
    Solve the difference equation

    2. Relevant equations
    y[n+2]-3y[n+1]+2y[n]==4n*2^n, n>=0
    y0=0,y1=1



    3. The attempt at a solution
    1. Homogenous solution:
    Characteristic equation => r^2-3r+2==0 <=> r1=1,r2=2 => yhn = C1*1^n+C2*2^n

    2. Particular solution:
    ypn = ?
    I tried different things but everything comes out wrong.

    I guess(what is the right word?) witha polynomial of the same degree as the right hand side of the equation how is it when 2^n is in both yhn and righthand side of the equation?
     
  2. jcsd
  3. Jan 2, 2012 #2
    Try ypn = bn(2^n). b is a constant to be found

    Notice in the complimentary function, (2^n) is multiplied by just a constant, this is different to the form of the particular solution as the RHS contains a non-zero first degree polynomial multiplying (2^n).

    But let's say for example your complementary function included a term cn(2^n) and the RHS = 4n(2^n) then for the particular solution we would try: Bn^2(2^n) so a 2nd degree polynomial multiplying 2^n
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook