初一上册代数题练习 急！！！！！！ 5
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已知X+XY=-2，XY-Y=-6求代数式/2X-{-4Y-3[（X+XY）的二次方+X]}-1/2[(Y-XY)的二次方-2Y]的值
解：原式=2x-{-4y-3[(x+xy)²+x]}-1/2[(y-xy)²-2y]
=2x+4y+3[(x+xy)²+x]-1/2（y-xy)²+y
=2x+4y+3(x+xy)²+3x-1/2(y-xy)²+y
=5x+5y+3(x+xy)²-1/2(y-xy)²
=5(x+y)+3(x+xy)²-1/2(y-xy)²
=5(x+y)+3(x+xy)²-1/2[-(xy-y)]²
∵(x+xy)-(xy-y)=x+xy-xy+y
=x+y
∴x+y=(-2)-(-6)
=4
当x+y=4,x+xy=-2,xy-y=-6时
原式=5×4+3×（-2）²-1/2[-(-6)]²
=20+12-18
=14
解：原式=2x-{-4y-3[(x+xy)²+x]}-1/2[(y-xy)²-2y]
=2x+4y+3[(x+xy)²+x]-1/2（y-xy)²+y
=2x+4y+3(x+xy)²+3x-1/2(y-xy)²+y
=5x+5y+3(x+xy)²-1/2(y-xy)²
=5(x+y)+3(x+xy)²-1/2(y-xy)²
=5(x+y)+3(x+xy)²-1/2[-(xy-y)]²
∵(x+xy)-(xy-y)=x+xy-xy+y
=x+y
∴x+y=(-2)-(-6)
=4
当x+y=4,x+xy=-2,xy-y=-6时
原式=5×4+3×（-2）²-1/2[-(-6)]²
=20+12-18
=14
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