x→0
lim (cotx-1/x)
=lim 1/tanx-1/x
=lim (x-tanx) / (xtanx)
该极限为0/0型,根据L'Hospital法则
=lim (x-tanx)' / (xtanx)'
=lim (1-1/cos^2x) / (tanx+x/cos^2)
=lim (cos^2x-1) / (sinxcosx+x)
该极限为0/0型,根据L'Hospital法则
=lim (cos^2x-1)' / (sinxcosx+x)'
=lim -2cosxsinx / (cos^2x-sin^2x+1)
=-lim 2sinx / 2cosx
=0
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