①a=1/2,b=1/3,则3a²-ab/3a²+5ab-2b²= ②若x²+xy-2y²=0,则x²+3xy+y²/x²+y

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①a=1/2,b=1/3,则3a²-ab/3a²+5ab-2b²= ②若x²+xy-2y²=0,则x²+3xy+y²/x²+y
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①a=1/2,b=1/3,则3a²-ab/3a²+5ab-2b²= ②若x²+xy-2y²=0,则x²+3xy+y²/x²+y
①a=1/2,b=1/3,则3a²-ab/3a²+5ab-2b²= ②若x²+xy-2y²=0,则x²+3xy+y²/x²+y

①a=1/2,b=1/3,则3a²-ab/3a²+5ab-2b²= ②若x²+xy-2y²=0,则x²+3xy+y²/x²+y
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x^2+xy-2y^2=0
(x-y)(x+2y)=0
x-y=0,x+2y=0
x=y或x=-2y

x=y,则原式=(y^2+3y^2+y^2)/(y^2+y^2)=5y^2/2y^2=5/2
x=-2y,则原式=(4y^2-6y^2+y^2)/(4y^2+y^2)=-y^2/5y^2=-1/5