∫(a→b) f'(3x) dx
= ∫(a→b) f'(3x) (1/3)d(3x)
= (1/3)f(3x) |(a→b)
= (1/3)[f(3b) - f(3a)]
或令u = 3x,du = 3 dx
∫(a→b) f'(3x) dx
= ∫(3a→3b) f'(u) * (1/3) du
= (1/3)f(u) |(3a→3b)
= (1/3)[f(3b) - f(3a)]