lim {f(Xo+h)+f(Xo-h)-2f(Xo)}/h^2
= lim {f′(Xo+h)-f′(Xo-h)}/2h
=lim {[f′(Xo+h)-f′(x0)]/h+[f′(Xo-h)-f′(x0)]/(-h)}
=2f″(x0)