连续用两次罗比达法则即可
lim [ e^(2x) - e^(-x) - 3x ] / (1-cosx)
=lim [2e^(2x) + e^(-x) -3] / sinx
=lim [4e^(2x) - e^x ] / cosx
= (4e^0 - e^0) / cos0
= 3