The definition of average velocity is:

According to this definition, the vector average velocity is obtained by dividing a vector by a scalar. Since the scalar () is always positive, this definition implies that the two vectors ( and ) have the same direction. We would like to apply this definition to find the velocity at point D. Then and in the definition correspond to an interval that spans point D, i.e., with an initial point just before point D and with a final point just after point D.

You indicated that the velocity of the object at point D is in direction 8. This means that the displacement vector in the definition above for the interval that spans point D also points in direction 8. Recall that the displacement for an interval is the vector whose tail is at the initial location and whose head is at the final location of the interval. The interval that spans point D has an initial location just prior to point D and a final location just after D. Does the displacement vector from the initial point to the final point really point in direction 8?