lane at $(3,-1)$. [Hint: Just as in the previous
    problem, find $Df(x,y)$, $dx$, $dy$, and $dz$. Then use
    differential notation.]
  </div>
</section>
                    
                    <section>
  <h1>Partial Derivatives of $z=f(x,y)$ functions</h1>
                        
                        <p>We can also
  understand tangent lines to surfaces using partial derivatives.
  The next problem will help you visualize what a partial
  derivative means in the graph of a surface.</p>

  <div cla