ice these new words in some problems.
  Remember, we're doing the exact same thing as we did at the start of this section.  We're just using the vocabulary of differentiation.</p>

                      
    <div class="instructor" id="estimating-change-with-diff">
Let's apply the
      problem and definition above. Fill in the blanks

      <ol>
        <li>If $f(x,y)=9-x^2-y^2$, then $df =
        f_xdx+f_ydy=\blank{1in}\,dx+\blank{1in}\,dy$. If we are on
        the surface at the point $(x,y,z)=(2,1,4)$