5a ) trivial
5b ) v(-x)' => (g o f)' = (g' o f) . f' g = v(x) f = -x => g' = v' et f' = -1
v(-x)' = v'(-x) * -1 = -v'(-x) = w'(-x)
v'(-x) = -v(-x) car 5a est vrai pour tout x
- [v(-x)]' = - v(-x) => w'(x) = w(x)