Solution (source code)

= Solution

Write $A=3+3/(2n)$. Since $\log Z_N$ contains $-NA\log\beta$,
$$
\boxed{
E=-\partial_\beta\log Z_N=NAk_BT,}
$$
$$
\boxed{
(\Delta E)^2=\partial_\beta^2\log Z_N
=NA(k_BT)^2,
\qquad C_V=NAk_B.}
$$
Consequently
$$
\frac{\Delta E}{E}=\frac1{\sqrt{NA}}\longrightarrow0
$$
in the thermodynamic <limit>: relative energy fluctuations vanish.