Solution
= Solution
Multiplying the inflaton equation by $\dot\phi$ gives
$$
\dot\rho=-3H\dot\phi^2.
$$
Comparison with $\dot\rho+3H(\rho+P)=0$ yields $\rho+P=\dot\phi^2$, and therefore
$$
\boxed{P=\frac12\dot\phi^2-V(\phi)}.
$$
= Solution
Multiplying the inflaton equation by $\dot\phi$ gives
$$
\dot\rho=-3H\dot\phi^2.
$$
Comparison with $\dot\rho+3H(\rho+P)=0$ yields $\rho+P=\dot\phi^2$, and therefore
$$
\boxed{P=\frac12\dot\phi^2-V(\phi)}.
$$