Question

若x(y-1)-y(x-1)=4,则xy-（x的平方+y的平方）/2=多少

请尽快，我在线等，拜托了

Answer 1

x(y-1)-y(x-1)=4
xy-x-xy+y=4
y-x=4
y=x+4
xy-(x^2+y^2)/2
=x(x+4)-[x^2+(x+4)^2]/2
=x^2+4x-(x^2+x^2+16+8x)/2
=x^2+4x-(x^2+8+4x)
=-8

Answer 2

x(y-1)-y(x-1)=4
xy-x-xy+y=4
y-x=4
(y-x)^2=16

xy-（x^2+y^2）/2
=-1/2(x^2+y^2-2xy)
=-1/2(x-y)^2
=-1/2*16
=-8