若A=x2-2xy+1/3y2，B=2x-3xy+y2，其中x=1，y=-2，求2A-B的值

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2A-B
=2(x2-2xy+1/3y2)-(2x-3xy+y2)
=2x2-4xy+2/3y2-2x+3xy-y2
=2x2-xy-y2+2/御明3y2
=2*1^2-1*(-2)-(-2)2+2/3(-2)2
=2+2-4+1/镇汪告6
=1/陵搭6

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