∀ {𝕜 : Type u_1} [inst : NontriviallyNormedField 𝕜] {E : Type u_2} [inst_1 : NormedAddCommGroup E]
  [inst_2 : NormedSpace 𝕜 E] {F : Type u_3} [inst_3 : NormedAddCommGroup F] [inst_4 : NormedSpace 𝕜 F] {x : E}
  {ι : Type u_4} {u : Finset ι} {A : ι → E → F},
  (∀ i ∈ u, DifferentiableAt 𝕜 (A i) x) → DifferentiableAt 𝕜 (∑ i ∈ u, A i) x