{bmatrix}?&amp;
      x^2+20\cos(5y)\end{bmatrix}\begin{bmatrix}dx\\dy\end{bmatrix}.$$</li>

      <li>If you hold $y$ constant (so $dy=0$), then what is
      $df/dx$? If you are at $(x,y)=(1,2)$, and if $x$ changes by
      $.2$, then about how much does $z=f(x,y)$ change?</li>

      <li>If you hold $x$ constant (so $dx=0$), then what is
      $df/dy$? If you are at $(x,y)=(1,2)$, and if $y$ changes by
      $-.3$, then about how much does $z=f(x,y)$ change?

            <li>If you are at the point $(x,y