lx+4l+lx-y+6l=0两项均是非负值,和为0,则它们都是0∴x+4=0,x-y+6=0∴x=-4,y=2∴x平方y-【2x平方y-(3xy-xy平方)-3x平方】=x²y-2x²y+(3xy-xy²)+3x²=-x²y+3xy-xy²+3x²=-xy(x+y)+3x(...