Let f : [a, b] → R be continuous on [a, b] and let v : [c, d] → R be differentiable on [c, d] with v ([c, d)] ⊂ [a, b]. If we define G(x) := ∫v(x)a f , show that Gʹx = f(v(x)) ∙ vʹ(x) for all x ∈ [c, d].