$$-\frac{1}{f}=\frac{1}{u}+\frac{1}{v}$$
$f=-\frac{uv}{u+v}$
$u\neq 0\text{ and }v\neq 0\text{ and }u\neq -v$
$u=-\frac{fv}{v+f}$
$v\neq 0\text{ and }f\neq 0\text{ and }f\neq -v$
