x(x-1)-(x^2-y)=-2化简，即x-y=2，则（x^2+y^2）/2-xy=1/2(x^2-2xy+y^2)=1/2(x-y)^2=1/2*2^2=2所以2分之x的2次方+y的2次方-xy的值为2
