v 9.4

v_{\pm} = \frac{v \pm w}{1 \pm \frac{v w}{c^{2}}} = \frac{\frac{c}{n} \pm w}{1 \pm \frac{w}{n c}}

${v_\pm=\frac{v \pm w}{1 \pm \frac{vw}{c^2}} =\frac{\frac{c}{n} \pm w}{1 \pm \frac{w}{nc}}}$

Error