f(a+h)=f(a)+f'(a)h+(1/2)f''(a)h^2+o(h^3)
f(a-h)=f(a)-f'(a)+(1/2)f''(a)h^2+o(h^3)
より
f''(a)=lim(h→0)(f(a-h)-2f(a)+f(a+h))/h^2