Question

如果x(x-1)-(x的2次方-y)=2求x的2次方+y的2次方/2-xy

Answer 1

x(x-1)-(x的2次方-y)=2
x²-x-x²+y=2
y-x=2
两边平方,
y²+x²-2xy=4
x²+y²=4+2xy

x的2次方+y的2次方/2-xy
=(4+2xy)/2 -xy
=2+xy-xy
=2

Answer 2

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