lim(x->0）(cosx) ^［（cotx） ^2］
=e^[lim (cotx)^2*lncosx]
=e^[lim (cotx)^2*ln(1+cosx-1)]
=e^[lim (cotx)^2*(cosx-1)]
=e^[lim (1/x^2)(-1/2)x^2]
=e^(-1/2)