题目

先化简,再求值: x2 + (-x2 +3xy +2y2)-(x2-xy +2y2),其中x=1,y=3. 答案：x2 + (-x2 +3xy +2y2)-(x2-xy +2y2) = x2-x2 +3xy +2y2-x2+xy-2y2 = 4xy-x2 当x=1,y=3时     4xy-x2=4×1×3-1=11｡

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