当x趋向0 (cotx－1/x)的极限
=lim(x->0)(cosx/sinx-1/x)
=lim(x->0)(xcosx-sinx)/xsinx
=lim(x->0)(xcosx-sinx)/x^2
=lim(x->0)(-xsinx)/2x
=lim(x->0)(-x^2)/2x
=0