(-x²+3xy-y²)-(-x²+4xy-y²)=-x²____________________+y²
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![(-x²+3xy-y²)-(-x²+4xy-y²)=-x²____________________+y²](/uploads/image/z/8653345-25-5.jpg?t=%EF%BC%88-x%26%23178%3B%2B3xy-y%26%23178%3B%29-%28-x%26%23178%3B%2B4xy-y%26%23178%3B%29%3D-x%26%23178%3B____________________%2By%26%23178%3B)
(-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