4x^2-4x+9y^2-12y+5=04x^2-4x+1+9y^2-12y+4=0（2x-1)^2+(3y-2)^2=0所以2x-1=0x=1/23y-2=0y=2/3(x-3y)^2-(x+3y)^2=(x-3y+x+3y)(x-3y-x-3y)=2x*(-6y)=-12xy=-12*1/2*2/3=-4