x平方+y平方-4x-2y+5=0
(x-2)^2+(y-1)^2=0
(x-2)^2=0
(y-1)^2=0
x=2
y=1
根号8y-2x分之（根号x）+y
=(根号2)+1/根号(8*1-2*2)
=[(根号2)+1]/2
或者=-[(根号2)+1]/2