※ 引述《bitchdog (PEaCE)》之銘言：
: Use the expression lim f(x+h)-f(x) to the deiative for
: h→0 ------------
: h
: the function f(x)= 3 at the point where x=2
: -----
: x+1
3/(x+h+1) - 3/(x+1)
f'(x)= lim ---------------------
h->0 h
-3 -3
= lim ----------------- = -------
h->0 (x+h+1)(x+1) (x+1)^2
-1
f'(2)= ---
3
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