※ 引述《zayin (看不清)》之銘言：
: 1.
: lim (cotx - 1/x)(cotx + 1/x) =?
: x→0+
: 乘開 = lim (cot^2 x - 1/x^2 )
: x→0+
tan(x) = x + x^3/3 + ...
1 1
lim (--------- - -----)
x→0+ tan^2 x x^2
x^2 - tan^2 x
= lim ---------------
x→0+ x^2 tan^2 x
x^2 - (x^2 + 2x^4/3 + ...)
= lim ---------------------------- = -2/3
x→0+ x^2(x^2 + 2x^4/3 + ...)
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