∀ {E : Type u_1} [inst : NormedAddCommGroup E] [inst_1 : NormedSpace ℝ E] {f : StrongDual ℝ E} {x : E} {t : ℝ},
  t < 0 → HasStrictFDerivAt (fun x => ‖x‖) f x → HasStrictFDerivAt (fun x => ‖x‖) (-f) (t • x)