1+2/[2^(-x)-1]
=1+2/(1/2^x-1),分母只有不断通分
=1+2/[(1-2^x)/2^x]
=1+2·2^x/(1-2^x)
=(1-2^x+2·2^x)/(1-2^x)
=(1+2^x)/(1-2^x)