$$\frac{\partial f}{\partial a}=cat+(\frac{a}{a-1})y-z$$
$\left\{\begin{matrix}c=-\frac{-ay+az-z}{at\left(1-a\right)}\text{, }&t\neq 0\text{ and }a\neq 1\text{ and }a\neq 0\\c\in \mathrm{R}\text{, }&\left(z=0\text{ and }a=0\right)\text{ or }\left(y=0\text{ and }z=0\text{ and }t=0\text{ and }a\neq 1\right)\text{ or }\left(a=\frac{z}{z-y}\text{ and }t=0\text{ and }y\neq 0\text{ and }y\neq z\right)\end{matrix}\right.$