(-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