※ 引述《bitchdog (PEaCE)》之銘言：
: Let f(x) = x(x+1)(x+2)...(x+n)
: -------------------- , find f'(0)
: cosx
lim f(x)-f(0) lim (x+1)(x+2)...(x+n)
f'(0)= ----------- = ------------------- = n!
x->0 x x->0 cosx
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