已知A=x三次方+2y三次方-xy二次方，B=-y三次方+x三次方+2xy二次方，其中x=三分之一，y=2，求a-b的值

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A-B=x³+2y³-xy²-(-y³+x³+2xy²)
=x³+2y³-xy²+y³-x³-2xy²
=3y³-3xy²
=3*2³-3*1/3*2²
=24-4
=20

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A-B=x³+2y³-xy²-(-y³+x³+2xy²)
=x³+2y³-xy²+y³-x³-2xy²
=3y³-3xy²
=3y²(y-x)
=3*2*2*(2-1/3)
=20

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