y ′ ′ + y ′ − 5 y = − 6 x 3 + 3 x 2 + 6 x y''+y'-5y=-6x^3+3x^2+6x y ′′ + y ′ − 5 y = − 6 x 3 + 3 x 2 + 6 x (1)
The solutions to a nonhomogeneous equation are of the form
y ( x ) = y h ( x ) + y p ( x ) y(x)= y_h(x) + y_p(x) y ( x ) = y h ( x ) + y p ( x ) ,
where y h y_h y h is the general solution to the associated homogeneous equation and y p y_p y p is a particular solution.
The associated homogeneous equation:
y ′ ′ + y ′ − 5 y = 0 y''+y'-5y=0 y ′′ + y ′ − 5 y = 0
The general solution of this equation is determined by the roots of the characteristic equation:
λ 2 + λ − 5 = 0 \lambda^2+\lambda-5=0 λ 2 + λ − 5 = 0
D = 1 2 − 4 ⋅ 1 ⋅ ( − 5 ) = 21 D=1^2-4\cdot1\cdot(-5)=21 D = 1 2 − 4 ⋅ 1 ⋅ ( − 5 ) = 21
λ = − 1 ± 21 2 \lambda=\frac{-1\pm\sqrt{21}}{2} λ = 2 − 1 ± 21
y h ( x ) = A e − 1 + 21 2 x + B e − 1 − 21 2 x y_h(x)=Ae^{\frac{-1+\sqrt{21}}{2}x}+Be^{\frac{-1-\sqrt{21}}{2}x} y h ( x ) = A e 2 − 1 + 21 x + B e 2 − 1 − 21 x , where A A A and B B B are constants.
The particular solution of the differential equation:
y p ( x ) = C x 3 + D x 2 + E x + F y_p(x)=Cx^3+Dx^2+Ex+F y p ( x ) = C x 3 + D x 2 + E x + F
y p ′ ( x ) = 3 C x 2 + 2 D x + E y_p'(x)=3Cx^2+2Dx+E y p ′ ( x ) = 3 C x 2 + 2 D x + E
y p ′ ′ ( x ) = 6 C x + 2 D y_p''(x)=6Cx+2D y p ′′ ( x ) = 6 C x + 2 D
Substitution y p y_p y p , y p ′ y_p' y p ′ , and y p ′ ′ y_p'' y p ′′ into (1) gives:
6 C x + 2 D + 3 C x 2 + 2 D x + E − 6Cx+2D+3Cx^2+2Dx+E- 6 C x + 2 D + 3 C x 2 + 2 D x + E − 5 ( C x 3 + D x 2 + E x + F ) = − 6 x 3 + 3 x 2 + 6 x 5(Cx^3+Dx^2+Ex+F)=-6x^3+3x^2+6x 5 ( C x 3 + D x 2 + E x + F ) = − 6 x 3 + 3 x 2 + 6 x
The last equation must be valid for all values of x, so the coefficients with the like powers of x
in the right and left sides must be identical:
{ − 5 C = − 6 3 C − 5 D = 3 6 C + 2 D − 5 E = 6 2 D + E − 5 F = 0 \begin{cases}
-5C=-6\\
3C-5D=3\\
6C+2D-5E=6\\
2D+E-5F=0
\end{cases} ⎩ ⎨ ⎧ − 5 C = − 6 3 C − 5 D = 3 6 C + 2 D − 5 E = 6 2 D + E − 5 F = 0
We find from this system that
C = 6 5 C=\frac65 C = 5 6 ; D = 3 25 D=\frac{3}{25} D = 25 3 ; E = 36 125 E=\frac{36}{125} E = 125 36 ; F = 66 625 F=\frac{66}{625} F = 625 66
As a result, the particular solution is written as
y p ( x ) = 6 5 x 3 + 3 25 x 2 + 36 125 x + 66 625 y_p(x)=\frac65x^3+\frac{3}{25}x^2+\frac{36}{125}x+\frac{66}{625} y p ( x ) = 5 6 x 3 + 25 3 x 2 + 125 36 x + 625 66
Answer: y ( x ) = A e − 1 + 21 2 x + B e − 1 − 21 2 x + 6 5 x 3 + y(x)=Ae^{\frac{-1+\sqrt{21}}{2}x}+Be^{\frac{-1-\sqrt{21}}{2}x}+\frac65x^3+ y ( x ) = A e 2 − 1 + 21 x + B e 2 − 1 − 21 x + 5 6 x 3 + 3 25 x 2 + 36 125 x + 66 625 \frac{3}{25}x^2+\frac{36}{125}x+\frac{66}{625} 25 3 x 2 + 125 36 x + 625 66 .
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