(D² - 2DD' + D'²)z = x²y²ex+y
"(D-D')^2z=0"
General solution:
C.F+P.I
C.F.:
"\\phi_1(y+x)+x\\phi_2(y+x)"
P.I.:
"\\frac{1}{(D-D')(D-D')}(x\u00b2y\u00b2e^x+y)"
"\\frac{1}{D-D'}(x\u00b2y\u00b2e^x+y)=\\int(x\u00b2y\u00b2e^x+y)dx=y^2(x^2-2x+2)e^x+xy"
"\\frac{1}{D-D'}(y^2(x^2-2x+2)e^x+xy)=\\int(y^2(x^2-2x+2)e^x+xy)dx="
"=y^2(x^2-4x+6)e^x+x^2y\/2"
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