lime^x-e^-x-2x/x-sinx x→0
=lim(e^x+e^(-x)-2)/(1-cosx) 连续多次用到洛必达法则
=lim(e^x-e^(-x))/sinx
=lim(e^x+e^(-x))/cosx
=lim(e^x+e^(-x))/limcosx
=2