已知y=3xy+x,求代数式x-2xy-y分之2x+3xy-2y的值

y=3xy+x
x-y=-3xy
x-2xy-y分之2x+3xy-2y
=(-3xy-2xy)分之(-6xy+3xy)
=-5xy分之(-3xy)
=5分之3这是降次法吗？y=3xy+xxy=(y-x)/3(x-2xy-y)分之(2x+3xy-2y)=[(x-y)+2(x-y)/3]分之[2(x-y)-(x-y)]=[5(x-y)/3]分之(x-y)=5分之3这是降次,但我觉得是刚才那样清晰,呵呵,不明白请追问y=3xy+x，那为什么xy=(y-x)/3y=3xy+x3xy=y-xxy=(y-x)/3