$(1)$ Show that $C_v=-T(\frac{de{l}^{2}A}{del{T}^2}{)}_V.$

$(2)$ Derive an expression for $(\frac{\text{d}\text{e}\text{l}\text{U}}{\text{d}\text{e}\text{l}\text{V}}{)}_T$ for a gas obeying the truncated virial equation $Z=1+\frac{B}{\frac{?}{\text{b}\text{a}\text{r}}(\frac{?}{\text{b}\text{a}\text{r}}(V))}.$