$$0.2s(4f-7)=0.\cos(10f-9)$$
$\left\{\begin{matrix}\\f=1.75\text{, }&\text{unconditionally}\\f\in \mathrm{C}\text{, }&s=0\end{matrix}\right.$
$\left\{\begin{matrix}\\s=0\text{, }&\text{unconditionally}\\s\in \mathrm{C}\text{, }&f=\frac{7}{4}\end{matrix}\right.$
$\left\{\begin{matrix}\\f=1.75\text{, }&\text{unconditionally}\\f\in \mathrm{R}\text{, }&s=0\end{matrix}\right.$
$\left\{\begin{matrix}\\s=0\text{, }&\text{unconditionally}\\s\in \mathrm{R}\text{, }&f=\frac{7}{4}\end{matrix}\right.$
