Here's what I've done:math

Basically I want to differentiate an expression once, then twice and finally use these two results to calculate the expression $$y''-2y'+2y = 4e^x\sin(x)$$ where $y(x) = xe^x(A\cos(x)+B\sin(x))$. My goal is to find constants $A,B$ such that the equality holds. Now using walfram alpha I get that $B=0$ and $A=-2$ as you can see from here: math2

Where did I go wrong with Mathematica? Or is it a mistake in my way of solving it? (consider that I've already found the solution to the homogeneous equation)

  • $\begingroup$ Wait I've messed up in one of the lines somehow $\endgroup$ – Euler_Salter Feb 4 '17 at 13:08
  • 3
    $\begingroup$ Please do not post images of your work, especially when the images display at a size that make them difficult to read. Please post your actual Mathematica code in the form of text that can be copied and pasted into a Mathematica notebook. Without such, it will be difficult to reproduce your problem and to experiment with possible solutions. $\endgroup$ – m_goldberg Feb 4 '17 at 14:07

Try this:

    y''[x] - 2 y'[x] + 2 y[x] - 4 Exp[x]*Sin[x] /. 
  y -> (#*Exp[#]*(A*Cos[#] + B*Sin[#]) &) // Simplify

(* 2 E^x (B Cos[x] - (2 + A) Sin[x]) *)

from where B=0 and A=-2.

Have fun!


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