若x(x-1)-(x²-y)=2,则 二分之一(x²+y²)-xy=

若x(x-1)-(x²-y)=2,则 二分之一(x²+y²)-xy=
若x(x-1)-(x²-y)=2,则 二分之一(x²+y²)-xy=

若x(x-1)-(x²-y)=2,则 二分之一(x²+y²)-xy=
x(x-1)-(x²-y)=2
x²-x-x²+y=2
y-x=2
(y-x)²=2²
x²-2xy+y²=4
1/2（x²+y²）-xy=2

x(x-1)-(x²-y)=2
x²-x-x²+y=2
y-x=2
(y-x)²=4
x²-2xy+y²=4
1/2(x²+y²)-xy=4÷2=2