e为底ln（limx→0(1-tan2x)^cscx）
L =lim(x→0)(1-tan2x)^cscxlnL =lim(x→0)ln(1-tan2x)/sinx (0/0) =lim(x→0)[-2(sec2x)^2/(1-tan2x)]/cosx =-2L =e^(-2)
