当x->0时,ln [1+ 2x/(1-x)] 2x/(1-x) 2x ,
lim(x->0) [1+ 2x/(1-x)] ^ (cotx)
= e ^ lim(x->0) cotx * ln [1+ 2x/(1-x)]
= e ^ lim(x->0) ln [1+ 2x/(1-x)] / tanx
= e ^ lim(x->0) 2 x / tanx
= e^2