计算(1/2xy+y²+1)+(x²-1/2xy-2y²-1)
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计算(1/2xy+y²+1)+(x²-1/2xy-2y²-1)
计算(1/2xy+y²+1)+(x²-1/2xy-2y²-1)
计算(1/2xy+y²+1)+(x²-1/2xy-2y²-1)
(1/2xy+y²+1)+(x²-1/2xy-2y²-1)
=1/2xy+y²+1+x²-1/2xy-2y²-1
=1/2xy-1/2xy+x²+y²-2y²-1+1
=x²-y²
(1/2xy+y²+1)+(x²-1/2xy-2y²-1) =x²-y²