计算：
-2a²b-3ab²+3ba²-4b²a
=a²b（-2+3）-ab²（3+4）
=a²b（1）-ab²（7）
=a²b-7ab²
=ab（a-7b)
xy-｛4x²y-［3x²y-(2xy-x²y)+3xy］｝
=xy-｛4x²y-［3x²y-2xy+x²y+3xy］｝
=xy-｛4x²y-［x²y（3+1）-xy（2-3）］｝
=xy-｛4x²y-［4x²y+xy］｝
=xy-｛4x²y-4x²y-xy｝
=xy-｛-xy｝
=xy+xy
=2xy
-2a²b-3ab²+3ba²-4b²a
=a^2b-7ab^2.
xy-｛4x²y-［3x²y-(2xy-x²y)+3xy］｝
=xy-{4x^2y-[3x^2y-2xy+x^2y+3xy]}
=xy-{4x^2y-[4x^2y+xy]}
=xy-{4x^2y-4x^2y-xy}
=xy+xy
=2xy.
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