">See 16.5: 27-30 for more
      practice.</span><span class="larsonfive">See Larson
      15.5:35&ndash;38 for more practice.</span>
    </div>Let $z=f(x,y)=9-x^2-y^2$. We'll parameterize this
    function by writing $x=x, y=y, z=9-x^2-y^2$, or in vector
    notation we'd write $$\vec r(x,y) = (x,y,f(x,y)).$$

    <ol>
      <li>Compute $\ds \frac{\partial \vec r}{\partial x}$ and $\ds
      \frac{\partial \vec r}{\partial y}$. Then evaluate these
      partials at $(x,y)=(2,1)$. What do these vectors mean