1+2^x=2^x(1+2^-x)
所以limln(1+2^x)ln(1+3/x)
=lim(ln2^x + ln(1+2^-x)) * ln(1+3/x)
=lim ln2^x * ln(1+3/x) + lim ln(1+2^-x) * ln(1+3/x)
=ln2*lim x*3/x + lim 2^-x * 3/x
=3ln2