（x-3)^2+│y+1│+z^2=0推出 x-3=0,x=3;
y+1=0,y=-1
z^2=0,z=0
则x^2-2xy+12xz-5x^2+3xy-z^2-8xz-2x^2
=x^2-2xy-5x^2+3xy-2x^2
=9+6-45-3-18
=-51