=3·lim(h→0)（f(x)-f(x+3h))/(3h)
令t=3h,则该极限= -3·lim(t→0) ( f(x+t) - f(x) )/ t
= -3·lim(t→0) ( f(x+t) - f(x) )/ (t-0)
= -3·f'(x)