∫[a→x] g(x)f(t) dt=g(x)∫[a→x] f(t) dt
因此(∫[a→x] g(x)f(t) dt)'
=( g(x)∫[a→x] f(t) dt )'
乘法求导公式
=g'(x)∫[a→x] f(t) dt+g(x)f(x)