x+y分之xy=2,则xy/(x+y)=2xy=2x+2y(-x+3xy-y)分之(3x-5xy+3y)=(3x-5xy+3y)/(-x+3xy-y)=[3x-5(2x+2y)+3y)/[-x+3(2x+2y)-y]=(3x-10x-10y+3y)/(-x+6x+6y-y)=(-7x-7y)/(5x+5y)=[-7(x+y)]/[5(x+y)]=-7/5=-1.4