因为x=-1/4,y=-1/2,所以：4xy-{(x²+5xy-y²)-(x²+3xy-2y²)}=4xy-(x²+5xy-y²-x²-3xy+2y²)=4xy-(2xy+y²)=2xy-y²=y(2x-y)=(-1/2)*[2*(-1/4)-(-1/2)]=(-1/2)*0=0