ition Df at all of the points.

o-------------------------------------------------o
|                                                 |
|             x  dx y  dy                         |
|             o---o o---o                         |
|              \  | |  /                          |
|               \ | | /                           |
|                \| |/         x y                |
|                 o=o-----------o                 |
|                  \           /                  |
|           