
      practice.</span>
    </div>
      
      Consider the function $f(x,y)=9-x^2-y^2$. Construct a
    3D surface plot of $f$.
    We'll focus on the point $(2,1)$.

    <ol>
      <li>Let $y=1$ and construct a graph in the $xz$ plane of the
      curve $z=f(x,1)=9-x^2-1^2$. Find an equation of the tangent
      line to this curve at $x=2$. Write the equation in the form
      $(z-z_0)=m(x-x_0)$ (find $z_0,m,x_0$). Also, find a direction
      vector $(1,0,?)$ for this line.</li>

      <li>Let $x=2$ and