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Mekhanik [1.2K]
3 years ago
15

What is the particular solution of this differential equation 5y'''-y''-6y'=1+x^2

Mathematics
1 answer:
leonid [27]3 years ago
5 0
Reduce the order of the ODE by setting z=y'[tex], so that [tex]z'=y'' and z''=y'''.

5z''-z'-6z=1+x^2

Consider the homogeneous ODE

5z''-z'-6z=0

which has characteristic equation

5r^2-r-6=(5r-6)(r+1)=0

which has roots at r=\dfrac65 and r=-1, so that the characteristic solution is

z_c=C_1e^{6x/5}+C_2e^{-x}

For the nonhomogeneous ODE,

5z''-z'-6z=1+x^2

we can expect a particular solution of the form

z_p=ax^2+bx+c
{z_p}'=2ax+b
{z_p}''=2a

Substituting these expressions into the ODE yields

10a-(2ax+b)-6(ax^2+bx+c)=1+x^2
\iff-6ax^2+(-2a-b)x+(10a-b-6c)=x^2+1

from which it follows that

\begin{cases}-6a=1\\-2a-b=0\\10a-b-6c=1\end{cases}\implies a=-\dfrac16,b=\dfrac13,c=-\dfrac12

and so the particular solution is

z_p=-\dfrac16x^2+\dfrac13x-\dfrac12

and the general solution for z is

z=z_c+z_p
z=C_1e^{6x/5}+C_2e^{-x}-\dfrac16x^2+\dfrac13x-\dfrac12

Integrate both sides once to solve for y:

z=y'\implies\displaystyle\int z\,\mathrm dx=\int y'\,\mathrm dx=y
\implies y=\hat{C_1}e^{6x/5}+\hat{C_2}e^{-x}-\dfrac19x^3+\dfrac16x^2-\dfrac12x+\hat{C_3}
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