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Marrrta [24]
3 years ago
8

Find a particular solution to the nonhomogeneous differential equation y'' + 4 y = cos(2x) + sin(2x).

Mathematics
1 answer:
alukav5142 [94]3 years ago
6 0
The characteristic solution follows from solving the characteristic equation,

r^2+4=0\implies r=\pm2i

so that

y_c=C_1\cos2x+C_2\sin2x

A guess for the particular solution may be a\cos2x+b\sin2x, but this is already contained within the characteristic solution. We require a set of linearly independent solutions, so we can look to

y_p=ax\cos2x+bx\sin2x

which has second derivative

{y_p}''=(-4ax+4b)\cos2x+(-4bx-4a)\sin2x

Substituting into the ODE, you have

y''+4y=\cos2x+\sin2x
\implies4b\cos2x-4a\sin2x=\cos2x+\sin2x
\implies\begin{cases}4b=1\\-4a=1\end{cases}\implies a=-\dfrac14,b=\dfrac14

Therefore the particular solution is

y_p=-\dfrac14x\cos2x+\dfrac14x\sin2x

Note that you could have made a more precise guess of

y_p=(a_1x+a_0)\cos2x+(b_1x+b_0)\sin2x

but, of course, any solution of the form a_0\cos2x+b_0\sin2x is already accounted for within y_c.
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