The corresponding homogeneous equation
y′′′+3y′′+3y′+y=0 Characteristic (auxiliary) equation
r3+3r2+3r+1=0
(r+1)3=0
r1=r2=r3=−1 The general solution of the homogeneous differential equation is
yh=c1e−x+c2xe−x+c3x2e−x Find the partial solution of the nonhomogeneous differential equation
y′′′+3y′′+3y′+y=xe−x−7y1(x)=x3(Ax+B)e−x+C
y1′(x)=(−Ax4−Bx3+4Ax3+3Bx2)e−x
y1′′(x)=(Ax4+Bx3−4Ax3−3Bx2)e−x
+(−4Ax3−3Bx2+12Ax2+6Bx)e−x
y1′′′(x)=(−Ax4−Bx3+8Ax3)e−x
+(6Bx2−12Ax2−6Bx)e−x
+(4Ax3+3Bx2−24Ax2)e−x
+(−12Bx+24Ax+6B)e−x Substitute
(−Ax4−Bx3+8Ax3)e−x
+(6Bx2−12Ax2−6Bx)e−x
+(4Ax3+3Bx2−24Ax2)e−x
+(−12Bx+24Ax+6B)e−x
+3(Ax4+Bx3−4Ax3−3Bx2)e−x
+3(−4Ax3−3Bx2+12Ax2+6Bx)e−x
+3(−Ax4−Bx3+4Ax3+3Bx2)e−x
+(Ax4+Bx3)e−x+C=xe−x−7
x4e−x:0=0
x3e−x:0=0
x2e−x:0=0
x1e−x:24A=1
x0e−x:B=0
x0:C=−7 The partial solution of the nonhomogeneous differential equation
y′′′+3y′′+3y′+y=xe−x−7 is
y1(x)=24x4e−x
Find the partial solution of the nonhomogeneous differential equation
y′′′+3y′′+3y′+y=xcosx
y2(x)=Axcosx+Bxsinx+Ccosx+Dsinx
y2′(x)=Acosx−Axsinx+Bsinx+Bxcosx
−Csinx+Dcosx
y2′′(x)=−2Asinx−Axcosx+2Bcosx
−Bxsinx−Ccosx−Dsinx
y2′′′(x)=−3Acosx+Axsinx−3Bsinx
−Bxcosx+Csinx−Dcosx Substitute
−3Acosx+Axsinx−3Bsinx
−Bxcosx+Csinx−Dcosx
−6Asinx−3Axcosx+6Bcosx
−3Bxsinx−3Ccosx−3Dsinx
+3Acosx−3Axsinx+3Bsinx+3Bxcosx
−3Csinx+3Dcosx
+Axcosx+Bxsinx+Ccosx+Dsinx
=xcosx
xcosx:2B−2A=1
xsinx:2B+2A=0
cosx:2D+6B−2C=0
sinx:−2C−6A−2D=0
A=−41,B=41,C=43,D=0The partial solution of the nonhomogeneous differential equation
y′′′+3y′′+3y′+y=xcosx is
y2(x)=−41xcosx+41xsinx+43cosx Find the partial solution of the nonhomogeneous differential equation
y′′′+3y′′+3y′+y=x2e−xsinx
y3(x)=(Ax2+Bx+C)e−x(Dcosx+Esinx)
y3′(x)=(2Ax+B)e−x(Dcosx+Esinx)
−(Ax2+Bx+C)e−x(Dcosx+Esinx)
+(Ax2+Bx+C)e−x(−Dsinx+Ecosx)
y3′′(x)=2Ae−x(Dcosx+Esinx)
−(4Ax+2B)e−x(Dcosx+Esinx)
+(4Ax+2B)e−x(−Dsinx+Ecosx)
+2(Ax2+Bx+C)e−x(Dsinx−Ecosx)
y3′′′(x)=−6Ae−x(Dcosx+Esinx)
+6Ae−x(−Dsinx+Ecosx)
+(12Ax+6B)e−x(Dsinx−Ecosx)
−2(Ax2+Bx+C)e−x(Dsinx−Ecosx)
+2(Ax2+Bx+C)e−x(Dcosx+Esinx) After substitution we have
y3(x)=(x2cosx−6xsinx−12cosx)e−x The general solution of the nonhomogeneous differential equation is
y=c1e−x+c2xe−x+c3x2e−x+24x4e−x−41xcosx+41xsinx+43cosx
+x2e−xcosx−6xe−xsinx−12e−xcosx
Comments