=Ix+2I+Ix-1I+Iy-5I+Iy+1I ≥Ix+2+x-1I+Iy-5+y+1I =I2x+1I+I2y-4I ≥I2x+2y-3I 所以-9≤2x+2y-3≤9 -3≤x+y≤6 所以x+y的最大值为6,最小值为-3
