x^7-1=(x-1)(1+x+x^2+cdots+x^6)x7−1=(x−1)(1+x+x2+⋯+x6)
x^5-1=(x-1)(1+x+x^2+cdots+x^4)x5−1=(x−1)(1+x+x2+⋯+x4)
so
(x^7-1)/(x^5-1)=(1+x+x^2+cdots+x^6)/(1+x+x^2+cdots+x^4)x7−1x5−1=1+x+x2+⋯+x61+x+x2+⋯+x4
then
lim_(x->1)log((x^7-1)/(x^5-1))=log(lim_(x->1)(1+x+x^2+cdots+x^6)/(1+x+x^2+cdots+x^4))=log(7/5)