>>880
{f(x)*g(x)}'
={f(x+h)g(x+h)-f(x)g(x)}/h
={f(x+h)g(x+h)-f(x+h)g(x)}/h + {f(x+h)g(x)-f(x)g(x)}/h
=f(x+h)*{g(x+h)-g(x)}/h + g(x)*{f(x+h)-f(x)}/h
→ f(x)*g'(x) + g(x)*f'(x)