Question #126113

(D^3-7DD'^2-6D'^3)z=cos(2x-3y)


1
Expert's answer
2020-07-12T15:05:50-0400

(D37DD26D3)z=cos(2x3y)(D^3-7DD'^2-6D'^3)z=cos(2x-3y)

Auxillary equation is:

m37m6=0m^3-7m-6=0

(m+2)(m+1)(m3)=0(m+2)(m+1)(m-3)=0

m=1,2,3m = -1, -2, 3

C.F.=f1(yx)+f2(y2x)+f3(y+3x)f_1(y-x)+f_2(y-2x)+f_3(y+3x)

P.I. =1D37DD26D3cos(2x3y)=\frac 1 {D^3-7DD'^2-6D'^3}cos(2x-3y)

Put D2=4,D2=9D^2 = -4, D'^2 = -9

P.I. =14D+63D+54Dcos(2x3y)== \frac 1 {-4D+63D+54D'}cos(2x-3y) = 159D+54Dcos(2x3y)\frac 1 {59D+54D'}cos(2x-3y)

=59D54D3481D22916D2cos(2x3y)=\frac {59D-54D'} {3481D^2-2916D'^2}cos(2x-3y) =59D54D=13924+26244cos(2x3y)= \frac {59D-54D'} {=13924+26244}cos(2x-3y)

=112320(59Dcos(2x3y)54Dcos(2x3y))=\frac 1 {12320}(59Dcos(2x-3y)-54D'cos(2x-3y))

=112320(118sin(2x3y)162sin(2x3y))=\frac 1 {12320}(-118sin(2x-3y)-162sin(2x-3y))

=28012320sin(2x3y)=7176sin(2x3y)=-\frac {280}{12320}sin(2x-3y)=-\frac{7}{176}sin(2x-3y)

The complete solution z=z= C.F.+P.I.=f1(yx)+f2(y2x)+f3(y+3x)=f_1(y-x)+f_2(y-2x)+f_3(y+3x) 7176sin(2x3y)-\frac{7}{176}sin(2x-3y)

Answer: z=f1(yx)+f2(y2x)+f3(y+3x)7176sin(2x3y)z=f_1(y-x)+f_2(y-2x)+f_3(y+3x)-\frac{7}{176}sin(2x-3y)


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