（x+2y-3y)- (-2x-y+xy ) 其中x+y=0.5 xy等于-0.5
问题描述：
（x+2y-3y)- (-2x-y+xy ) 其中x+y=0.5 xy等于-0.5
答
（x+2y-3y)- (-2x-y+xy )=3x-xy=3x+0.5
由x+y=0.5 xy=-0.5知道x是方程
x²-0.5x-0.5=0的解
∴x=-0.5或1
∴原式=-1或3.5
答
（x+2y-xy)- (-2x-y+xy )
=x+2y-xy+2x+y-xy
=3x+3y-2xy
=3(x+y)-2xy
=3×0.5-2×(-0.5)
=1.5+1
=2.5