Solution
= Solution
Using $v/c=\tanh\theta$ and the <hyperbolic tangent> addition identity,
$$
\frac{v''}{c}=\tanh(\theta+\theta')=\frac{v/c+v'/c}{1+vv'/c^2}.
$$
Consequently the <relativistic velocity addition> rule is
$$
\boxed{v''=\frac{v+v'}{1+vv'/c^2}=\frac{c^2(v+v')}{c^2+vv'}}.
$$