原式=e^[lim(x->0)(lncosx)/x]=e^[lim(x->0)(1/cosx ×(-sinx))/1]=e^[lim(x->0)-tanx]=e^0=1
(cosx)^1/x ?就是（cosx）/x ？lim (cosx)/x=lim(-sinx)/1|x=0 =0
