English
If f and g are differentiable at x, then fderiv of (f+g) at x equals sum of fderiv of f and g at x.
Русский
Если f и g дифференцируемы в x, то производная от суммы f+g в x равна сумме производных.
LaTeX
$$$\displaystyle fderiv 𝕜 (f+g) x = fderiv 𝕜 f x + fderiv 𝕜 g x.$$$
Lean4
theorem fderiv_fun_add (hf : DifferentiableAt 𝕜 f x) (hg : DifferentiableAt 𝕜 g x) :
fderiv 𝕜 (fun y => f y + g y) x = fderiv 𝕜 f x + fderiv 𝕜 g x :=
(hf.hasFDerivAt.add hg.hasFDerivAt).fderiv