 of the book). Don't use the formulas on pages 925&ndash;930.
      Instead, use matrix multiplication. The formulas are just a
      way of writing matrix multiplication without writing down the
      matrices, and only work for functions from $\RR^n\to\RR$. Our
      matrix multiplication method works for any function from
      $\RR^n\to\RR^m$.</span>
    </div><div class="hidden">In class, I also replace $x$ and $y$ in
    $f=x^2+xy$ with what they are in terms of $t$, and then use
    first-semester ca