      <li>If the current measurements of a box are $x=2$, $y=3$,
      and $z=5$, and we know that $x$ increases by .01, $y$
      increases by .02, and $z$ decreases by .03, then by about how
      much will the volume change? Use your formula for $dV$ to
      approximate this.</li>
    </ol>
  </div>


                    <p>Part 4 in each problem above, expressing relationships
  between changes in terms of differentials, is the KEY idea, let
  me repeat, THE KEY IDEA, to the rest of this course. The
  