f(g(x+Δx)) = f( g(x) + g'(x)·Δx+o(Δx) ) = #
  f(t+Δ) = f(t) + f'(t)·Δt + o(Δt),  t = g(x),  Δt = g'(x)·Δx
# = f(g(x)) + f'(g(x))g'(x)·Δx + o(g'(x)·Δx) = #
  o(k·Δx) = o(Δx)
# = f(g(x)) + f'(g(x))g'(x)·Δx + o(Δx)