ase of $1$ unit in the $x$ direction will increase
      $y$ by $m$ units. Fill in the blanks by using the slopes of
      tangent lines calculated above for the function
      $z=f(x,y)=9-x^2-y^2$.

        <ul>
          <li>Increasing $x$ by 1 unit when $y$ does not change
          will cause $z$ to increase by about ?
          units.</li>

          <li>Increasing $y$ by 1 unit when $x$ does not change
          will cause $z$ to increase by about ?
          units.</li>

          <li>Increasing $x$ 