题目是不是x(x-1)-(x²-y)=4,求(x²+y²)/2-xy呢？如果是的话则x(x-1)-(x²-y)=4 即x²-x-x²+y=4，即x-y= -4 (x²+y²)/2-xy=(x²+y²-2xy)/2=(x-y)²/2=(-4)²/2=8
