Solution
= Solution
At fixed volume, a reversible infinitesimal heating supplies $\delta Q_{\rm rev}=C_V\,dT$. Since <entropy> satisfies $dS=\delta Q_{\rm rev}/T$,
$$
\Delta S=C_V\int_{T_i}^{T_f}\frac{dT}{T}
=\boxed{C_V\log\frac{T_f}{T_i}}.
$$
= Solution
At fixed volume, a reversible infinitesimal heating supplies $\delta Q_{\rm rev}=C_V\,dT$. Since <entropy> satisfies $dS=\delta Q_{\rm rev}/T$,
$$
\Delta S=C_V\int_{T_i}^{T_f}\frac{dT}{T}
=\boxed{C_V\log\frac{T_f}{T_i}}.
$$