Equation 15.20.6B34A7F7705B9

\begin{equation} \mathrm{d}\bigl (\Phi (r,h(r))\bigr ) =\bigl (k(r)-A(r,h(r))z(r) +\partial _ r\Phi (r,h(r))\bigr )\, \mathrm{d}r. \end{equation}