Solution
= Solution
When $0\leq v,v'\ll c$, expand the denominator:
$$
v''=v+v'-\frac{vv'(v+v')}{c^2}+O\!\left(\frac{v^2v'^2(v+v')}{c^4}\right).
$$
The leading term is ordinary <Galilean velocity addition>. When both $v,v'\to c$ from below, the exact formula instead gives $\boxed{v''\to c}$, rather than $2c$. Indeed, writing $a=v/c$, $b=v'/c$, we have $1-v''/c=(1-a)(1-b)/(1+ab)>0$ whenever $a,b<1$.