(D3−7DD′2−6D′3)z=cos(2x−3y)
Auxillary equation is:
m3−7m−6=0
(m+2)(m+1)(m−3)=0
m=−1,−2,3
C.F.=f1(y−x)+f2(y−2x)+f3(y+3x)
P.I. =D3−7DD′2−6D′31cos(2x−3y)
Put D2=−4,D′2=−9
P.I. =−4D+63D+54D′1cos(2x−3y)= 59D+54D′1cos(2x−3y)
=3481D2−2916D′259D−54D′cos(2x−3y) ==13924+2624459D−54D′cos(2x−3y)
=123201(59Dcos(2x−3y)−54D′cos(2x−3y))
=123201(−118sin(2x−3y)−162sin(2x−3y))
=−12320280sin(2x−3y)=−1767sin(2x−3y)
The complete solution z= C.F.+P.I.=f1(y−x)+f2(y−2x)+f3(y+3x) −1767sin(2x−3y)
Answer: z=f1(y−x)+f2(y−2x)+f3(y+3x)−1767sin(2x−3y)
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