gnAdyZHLZK5k4GMh4HeL5ojnmvIleVqQhciwWXzHxoXOBZQdf_PUtO4phMKfqJ6TJl49XCPyfYlynt6LKQu3QafGHznitNUdjTfHiwp-jWK_SdH73oropfpKp5u5d1yljF_YDmjyLHioPZ8n5jMCn7LDMSdtsgoHfmROy-BN9Vrcb">
      Sage</a> for a picture.
    </div>

    <div class="marginpar">
      <span class="bmw">See 14.6: 9-12 for more
      practice.</span><span class="larsonfive">See Larson
      13.7:17&ndash;30 for more practice.</span>
    </div>Let $z=f(x,y)=9-x^2-y^2$.

    <ol>
      <li>The derivative is $Df(x,y) =
      \begin{bmatrix}-2x&a