x^2-2x+y^2-4y+5=0
(x^2-2x+1)+(y^2-4y+4)=0
(x-1)^2+(y-2)^2=0
所以x-1=0,y-2=0
x=1,y=2
9x^2y^2-12x^3y^2+4x^4y^2
=x^2y^2(9-12x+4x^2)
=x^2y^2(2x-3)^2
=1*4*(2-3)^2
=4