Question #284329

y'''' + 3y''' + 3y' + y = x*e^(-x) + x*cosx - 7 + x^(2) *e^(-x) *sinx


1
Expert's answer
2022-01-03T16:29:15-0500

The corresponding homogeneous equation


y+3y+3y+y=0y''' + 3y'' + 3y' + y =0

Characteristic (auxiliary) equation


r3+3r2+3r+1=0r^3+3r^2+3r+1=0

(r+1)3=0(r+1)^3=0

r1=r2=r3=1r_1=r_2=r_3=-1

The general solution of the homogeneous differential equation is


yh=c1ex+c2xex+c3x2exy_h=c_1e^{-x}+c_2xe^{-x}+c_3x^2e^{-x}

Find the partial solution of the nonhomogeneous differential equation


y+3y+3y+y=xex7y''' + 3y'' + 3y' + y =xe^{-x} - 7y1(x)=x3(Ax+B)ex+Cy_1(x)=x^3(Ax+B)e^{-x}+C

y1(x)=(Ax4Bx3+4Ax3+3Bx2)exy_1'(x)=(-Ax^4-Bx^3+4Ax^3+3Bx^2)e^{-x}

y1(x)=(Ax4+Bx34Ax33Bx2)exy_1''(x)=(Ax^4+Bx^3-4Ax^3-3Bx^2)e^{-x}

+(4Ax33Bx2+12Ax2+6Bx)ex+(-4Ax^3-3Bx^2+12Ax^2+6Bx)e^{-x}

y1(x)=(Ax4Bx3+8Ax3)exy_1'''(x)=(-Ax^4-Bx^3+8Ax^3)e^{-x}

+(6Bx212Ax26Bx)ex+(6Bx^2-12Ax^2-6Bx)e^{-x}




+(4Ax3+3Bx224Ax2)ex+(4Ax^3+3Bx^2-24Ax^2)e^{-x}

+(12Bx+24Ax+6B)ex+(-12Bx+24Ax+6B)e^{-x}

Substitute


(Ax4Bx3+8Ax3)ex(-Ax^4-Bx^3+8Ax^3)e^{-x}

+(6Bx212Ax26Bx)ex+(6Bx^2-12Ax^2-6Bx)e^{-x}


+(4Ax3+3Bx224Ax2)ex+(4Ax^3+3Bx^2-24Ax^2)e^{-x}

+(12Bx+24Ax+6B)ex+(-12Bx+24Ax+6B)e^{-x}




+3(Ax4+Bx34Ax33Bx2)ex+3(Ax^4+Bx^3-4Ax^3-3Bx^2)e^{-x}

+3(4Ax33Bx2+12Ax2+6Bx)ex+3(-4Ax^3-3Bx^2+12Ax^2+6Bx)e^{-x}

+3(Ax4Bx3+4Ax3+3Bx2)ex+3(-Ax^4-Bx^3+4Ax^3+3Bx^2)e^{-x}

+(Ax4+Bx3)ex+C=xex7+(Ax^4+Bx^3)e^{-x}+C =xe^{-x} - 7

x4ex:0=0x^4e^{-x}:0=0

x3ex:0=0x^3e^{-x}:0=0

x2ex:0=0x^2e^{-x}:0=0

x1ex:24A=1x^1e^{-x}:24A=1

x0ex:B=0x^0e^{-x}:B=0

x0:C=7x^0:C=-7

The partial solution of the nonhomogeneous differential equation


y+3y+3y+y=xex7y''' + 3y'' + 3y' + y =xe^{-x} - 7

is

y1(x)=x4ex24y_1(x)=\dfrac{x^4e^{-x}}{24}

Find the partial solution of the nonhomogeneous differential equation


y+3y+3y+y=xcosxy''' + 3y'' + 3y' + y =x\cos x

y2(x)=Axcosx+Bxsinx+Ccosx+Dsinxy_2(x)=Ax\cos x+Bx\sin x+C\cos x+D\sin x


y2(x)=AcosxAxsinx+Bsinx+Bxcosxy_2'(x)=A\cos x-Ax\sin x+B\sin x+Bx\cos x

Csinx+Dcosx-C\sin x+D\cos x

y2(x)=2AsinxAxcosx+2Bcosxy_2''(x)=-2A\sin x-Ax\cos x+2B\cos x

BxsinxCcosxDsinx-Bx\sin x-C\cos x-D\sin x

y2(x)=3Acosx+Axsinx3Bsinxy_2'''(x)=-3A\cos x+Ax\sin x-3B\sin x

Bxcosx+CsinxDcosx-Bx\cos x+C\sin x-D\cos x

Substitute


3Acosx+Axsinx3Bsinx-3A\cos x+Ax\sin x-3B\sin x

Bxcosx+CsinxDcosx-Bx\cos x+C\sin x-D\cos x

6Asinx3Axcosx+6Bcosx-6A\sin x-3Ax\cos x+6B\cos x

3Bxsinx3Ccosx3Dsinx-3Bx\sin x-3C\cos x-3D\sin x

+3Acosx3Axsinx+3Bsinx+3Bxcosx+3A\cos x-3Ax\sin x+3B\sin x+3Bx\cos x

3Csinx+3Dcosx-3C\sin x+3D\cos x

+Axcosx+Bxsinx+Ccosx+Dsinx+Ax\cos x+Bx\sin x+C\cos x+D\sin x

=xcosx=x\cos x

xcosx:2B2A=1x\cos x: 2B-2A=1

xsinx:2B+2A=0x\sin x: 2B+2A=0

cosx:2D+6B2C=0\cos x: 2D+6B-2C=0

sinx:2C6A2D=0\sin x: -2C-6A-2D=0

A=14,B=14,C=34,D=0A=-\dfrac{1}{4}, B=\dfrac{1}{4}, C=\dfrac{3}{4}, D=0

The partial solution of the nonhomogeneous differential equation


y+3y+3y+y=xcosxy''' + 3y'' + 3y' + y =x\cos x

is


y2(x)=14xcosx+14xsinx+34cosxy_2(x)=-\dfrac{1}{4}x\cos x+\dfrac{1}{4}x\sin x+\dfrac{3}{4}\cos x

Find the partial solution of the nonhomogeneous differential equation


y+3y+3y+y=x2exsinxy''' + 3y'' + 3y' + y =x^2 e^{-x}\sin x

y3(x)=(Ax2+Bx+C)ex(Dcosx+Esinx)y_3(x)=(Ax^2+Bx+C)e^{-x}(D\cos x+E\sin x)


y3(x)=(2Ax+B)ex(Dcosx+Esinx)y_3'(x)=(2Ax+B)e^{-x}(D\cos x+E\sin x)

(Ax2+Bx+C)ex(Dcosx+Esinx)-(Ax^2+Bx+C)e^{-x}(D\cos x+E\sin x)

+(Ax2+Bx+C)ex(Dsinx+Ecosx)+(Ax^2+Bx+C)e^{-x}(-D\sin x+E\cos x)

y3(x)=2Aex(Dcosx+Esinx)y_3''(x)=2Ae^{-x}(D\cos x+E\sin x)

(4Ax+2B)ex(Dcosx+Esinx)-(4Ax+2B)e^{-x}(D\cos x+E\sin x)

+(4Ax+2B)ex(Dsinx+Ecosx)+(4Ax+2B)e^{-x}(-D\sin x+E\cos x)

+2(Ax2+Bx+C)ex(DsinxEcosx)+2(Ax^2+Bx+C)e^{-x}(D\sin x-E\cos x)

y3(x)=6Aex(Dcosx+Esinx)y_3'''(x)=-6Ae^{-x}(D\cos x+E\sin x)

+6Aex(Dsinx+Ecosx)+6Ae^{-x}(-D\sin x+E\cos x)

+(12Ax+6B)ex(DsinxEcosx)+(12Ax+6B)e^{-x}(D\sin x-E\cos x)

2(Ax2+Bx+C)ex(DsinxEcosx)-2(Ax^2+Bx+C)e^{-x}(D\sin x-E\cos x)

+2(Ax2+Bx+C)ex(Dcosx+Esinx)+2(Ax^2+Bx+C)e^{-x}(D\cos x+E\sin x)

After substitution we have


y3(x)=(x2cosx6xsinx12cosx)exy_3(x)=(x^2\cos x-6x\sin x-12\cos x)e^{-x}

The general solution of the nonhomogeneous differential equation is


y=c1ex+c2xex+c3x2exy=c_1e^{-x}+c_2xe^{-x}+c_3x^2e^{-x}+x4ex2414xcosx+14xsinx+34cosx+\dfrac{x^4e^{-x}}{24}-\dfrac{1}{4}x\cos x+\dfrac{1}{4}x\sin x+\dfrac{3}{4}\cos x

+x2excosx6xexsinx12excosx+x^2e^{-x}\cos x-6xe^{-x}\sin x-12e^{-x}\cos x


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