nts in the $xy$ plane that lie on the
        level curves you drew above. At these 8 points, add the
        vector given by the derivative evaluated at that point. For
        example, at $(0,0)$, draw the vector $Df(0,0)=(0,1)$, and
        at the point $(1,1)$, draw the vector $Df(1,1)=(-2,1)$.
        What do you observe about the relationship between the
        vectors and the contour lines?
      </li>
    </ol>
  </div>
</section>
                
                
                <section>
  <h1>Th