x(y-1)-y(x-1)=4xy-x-xy+y=4-x+y=4x-y=-4(x²+y²)/2-xy=(x²-2xy+y²)/2=(x-y)²/2=(-4)²/2=8
x（y-1)-y(x-1)=4xy-x-yx+y=4y-x=4...y2/2什么哦
等于8
