设f(x)在x=2处可导,且f'(2)=1,则lim h→0 [ f(2+h)-f(2-h)]/h等于多少,

问题描述:

设f(x)在x=2处可导,且f'(2)=1,则lim h→0 [ f(2+h)-f(2-h)]/h等于多少,

由导数的定义可知f(x)在x=2处可导,且f'(2)=1,就是说lim(f(2+h)-f(2))/h=1
于是,lim [ f(2+h)-f(2-h)]/h
= lim [ f(2+h)-f(2)+f(2)-f(2-h)]/h
=lim (f(2+h)-f(2))/h+(f(2-h)-f(2))/(-h)
=f'(2)+f'(2)
=1+1
=2