（-x²+3xy-y²)-(-x²+4xy-y²)=-x²____________________+y²

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（-x²+3xy-y²)-(-x²+4xy-y²)=-x²____________________+y²（-x²+3xy-y

（-x²+3xy-y²)-(-x²+4xy-y²)=-x²____________________+y²
（-x²+3xy-y²)-(-x²+4xy-y²)=-x²____________________+y²

（-x²+3xy-y²)-(-x²+4xy-y²)=-x²____________________+y²
（-x²+3xy-y²)-(-x²+4xy-y²)
=-x²+3xy-y²+x²-4xy+y²
=-xy
=-x²__+x²-xy-y²__________________+y²

原式=-xy=-x²+x²-xy-y²+y²，∴横线处为x²-xy-y²