[f(x)g(x)]'=limh→0f(x+h)g(x+h)−f(x)g(x)h
=limh→0f(x+h)g(x)−f(x)g(x+h)g(x+h)g(x)h
=limh→0f(x+h)g(x)−f(x)g(x+h)hg(x+h)g(x)
=limh→0f(x+h)g(x)−f(x)g(x)−f(x)g(x+h)+f(x)g(x)hg(x+h)g(x)
=limh→0f(x+h)−f(x)hg(x)−f(x)g(x+h)−g(x)hg(x+h)g(x)
=f'(x)g(x)−f(x)g'(x)[g(x)]2