. We will be dealing
  with nice functions of this sort in this class, so we will have
  this relationship between $f_{xy}$ and $f_{yx}$.

  <div class="problem">
    Let $z=f(x,y)=9-x^2-y^2$. We'll look at the point $(2,1)$, like
    above.

    <ol>
      <li>Compute $f_{xx}$, $f_{yy}$, $f_{xy}$, and $f_{yx}$ at the
      point $(2,1)$.</li>

      <li>How do you interpret each of those second partials
      graphically on this function?</li>
    </ol>
  </div>
<div class="problem">Let $f(x,y)=3xy^2+y\ln 