Solution
= Solution
Differentiate the <Friedmann equation>:
$$
2H\dot H
=\frac{1}{3M_{\rm Pl}^2}
\left(\dot\phi\,\ddot\phi+c^2V'(\phi)\dot\phi\right).
$$
The scalar-field equation gives $\ddot\phi+c^2V'=-3H\dot\phi$, hence
$$
2H\dot H=-\frac{H\dot\phi^2}{M_{\rm Pl}^2}.
$$
By continuity this also holds through isolated zeros of $H$, so the <Raychaudhuri equation> is
$$
\boxed{\ \dot H=-\frac{\dot\phi^2}{2M_{\rm Pl}^2}\ }.
$$