原式=3x2y-[2xy2-(2xy-3x2y)+xy]+3xy2=3x2y-(2xy2-2xy+3x2y+xy)+3xy2=3x2y-2xy2+2xy-3x2y-xy+3xy2=xy+xy2,当x= 1 3 ,y=-5时,原式= 1 3 ×(-5)+ 1 3 ×25= 20 3 .