{\dot{z}}^{'} = \frac{c - u}{1 - \frac{c u}{c^{2}}} = \frac{c - u}{\frac{c (c - u)}{c^{2}}} = c

${\dot z'=\frac{c-u}{1- \frac{cu}{c^2}}=\frac{c-u}{\frac{c(c-u)}{c^2}} = c}$

Error