因为x的平方+4（y的平方）-8x+12y+5
=x^2+4y^2-8x+12y+5
=x^2-8x+16+4y^2+12y+9-20
=(x-4)^2+(2y+3)^2-20
所以当x=4,y=-3/2时,有最小值为-20