4x^2+9y^2-12x+6y+9
=4x^2-12x+9+9y^2+6x+1-1
=4(x-3/2)^2+9(y+1/3)^2-1
>=-1
当x=3/2,y=-1/3时4x^2+9y^2-12x+6y+9的最小值-1