$8-6.$ Show that

$(\frac{\text{d}\text{e}\text{l}(\frac{?}{\text{b}\text{a}\text{r}}(U))}{\text{d}\text{e}\text{l}(\frac{?}{\text{b}\text{a}\text{r}}(V))}{)}_T=\frac{R{T}^2}{\frac{?}{\text{b}\text{a}\text{r}}(V{)}^2}\frac{d{B}_{2V}}{dT}+\frac{R{T}^2}{\frac{?}{\text{b}\text{a}\text{r}}(V{)}^3}\frac{d{B}_{3V}}{dT}...$