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)