解：k=(y1-y2)/(x1-x2)=(lnx1-lnx2)/(x1-x2)=(x1+x2)* (lnx1-lnx2)/(x1-x2)= (1+x2/x1)* (ln(x1/x2))/(1-x2/x1),令 x2/x1=n， (1+x2/x1)/(1-x2/x1)=r,r<0，那么 x1/x2=(r+1)/(r-1)=1-2/(r-1)，那么 ，k(x1+x2)=ln((1-2/(r-1))^r
