4x^2+9y^2-4x+12y+5
=4x^2-4x+1+9y^2+12y+4
=(2x-1)²+(3y+2)²=0
2x-1=0,3y+2=0
x=1/2
y=-2/3
解集可表示为{(x,y)|x=1/2,y=-2/3}