lim（h→0）[f(a+2 h)-2f(a+h)+f(a)]/h^2
=lim（h→0）{f(a+2 h)-f(a+h)-[f(a+h)-f(a)]}/h^2
=lim（h→0）[f'(a+h)-f'(a)]/h
=f''(a)