xx+yy-4x+6y+28
=(x^2-4x+4)+(y^2+6y+9)+15
=(x-2)^2+(y+3)^2+15
因为(x-2)^2≥0,(y+3)^2≥0,
所以当(x-2)^2=0,(y+3)^2=0时,即x=2,y=-3时,最小值是15