# Solving an ODE with eulers formula.

by cragar
Tags: eulers, formula, solving
 P: 2,466 If I have $y''+y'+2y=sin(x)+cos(x)$ can I just say $y=Ae^{ix}$ and then find y' and y'' and then plug them in and solve for A. so I get that $A= \frac{1}{1+i}$ then i multiply and divided by the complex conjugate. then I back substitute in Eulers formula. now since I have my original equation has both a real and an imaginary part. when I multiply out A times Eulers formula, I will take both real and imaginary parts. and I get that y=sin(x) and tested this and it works. But is what i did with Eulers formula ok.
HW Helper
Thanks
P: 26,148
hi cragar!
 Quote by cragar If I have $y''+y'+2y=sin(x)+cos(x)$ can I just say $y=Ae^{ix}$
not really

for a particular solution for an RHS of sinx + cosx, you should use Aeix + Be-ix

(or Ccosx + Dsinx)
in this case, you're lucky that B = 0
(but don't do it again! )
 P: 2,466 if i substitute what you said. Do I take the real or imaginary part in the end. Or do I just take it all.
 Sci Advisor HW Helper Thanks P: 26,148 Solving an ODE with eulers formula. you take it all but if you're expecting a real solution anyway, you might as well use Ccosx + Dsinx in the first place! (technically, C and D are both complex, but they'll come out real)
 P: 2,466 thanks for your answers. I like to push the limits of Eulers formula. And see what it can do.

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