∫(0,t+h) G(t+h-s)f(s)ds - ∫(0,t) G(t-s)f(s)ds
= ∫(t,t+h) G(t+h-s)f(s)ds + ∫(0,t) G(t+h-s)f(s)ds - ∫(0,t) G(t-s)f(s)ds
= h G((1-u)h)f(t+uh) + ∫(0,t) {G(t+h-s) - G(t-s)}f(s)ds  (0<u<1)

d/dt[∫(0,t) G(t-s)f(s)ds]
= lim[h→0] [∫(0,t+h) G(t+h-s)f(s)ds - ∫(0,t) G(t-s)f(s)ds]/h
= lim[h→0] [G((1-u)h)f(t+uh) + ∫(0,t) {G(t+h-s) - G(t-s)}/h * f(s)ds]
= G(0)f(t) + ∫(0,t) G'(t-s)f(s)ds