Suppose that h > 0 and f' exists and is finite for every x
in (a-h,a+h). Also suppose that f is continuous on [a-h,a+h].
If f"(a) exists, show that the limit
f(a+h) - 2f(a) + f(a-h)
lim ------------------------- = f"(a)
h→0 h^2
Note: 請順便給一個反例 f(x), 使得左邊的極限式存在, 但 f" 不存在.
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