（7）若x(y-1)-y(x-1)=4,求x²+y²/2-xy的值

x(y-1)-y(x-1)=4
xy-x-xy+y=4
-x+y=4
∴x-y=-4
(x²+y²)/2-xy=(x²-2xy+y²)/2=(x-y)²/2=8
a=1/20 x+20,b=1/20 x+19,c=1/20 x+21
∴a-b=1
b-c=-2
c-a=1
a²+b²+c²-ab-bc-ac
=1/2[(a-b)²+(b-c)²+(c-a)²]
=1/2[1²+(-2)²+1²]
=3