用罗比达法则：
lim(ln(1+2^x))/(ln(1+3^x))
=lim[2^xln2/(1+2^x)][3^xln3/(1+3^x)]
=lim (ln2/ln3) [(2^x+6^x)/(3^x+6^x)] (分子分母同除以6^x)
=ln2/ln3