Question

x(y-1)-y(x-1)=4,则x的平方+y的平方除以2-xy=

Answer 1

x(y-1)-y(x-1)
=xy-x-xy+y
=y-x=4
而(x平方+y平方)/2-xy=(x平方+y平方-2xy)/2=(y-x)平方/2=16/2=8

Answer 2

等于7

追问

为什么？

追答

x(y-1)-y(x-1)=4 : y-x=4;
(x*x+y*y)/2-xy=(x-y)*(x-y)/2=8,不好意思，因该等于8