看过程
=(x→0)lim[1/(xsinx)-1/(xsinxcosx)]
=(x→0)lim[(cosx-1)/(xsinxcosx)]
=(x→0)lim[(2cosx-2)/(xsin2x)]
(0/0型，分子分母同时求导)
=(x→0)lim[(-2sin2x)/(sin2x+2xcos2x)]
(0/0型，分子分母同时求导)
=(x→0)lim[(-4cos2x)/(2cos2x+2cos2x-4xsin2x)]
(x→0)lim[(-4cos2x)/(4cos2x-4xsin2x)]
=-4/4
=-1
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