lim{n->inf}[6^(n+1)*(n+1)/[7^(n+1)-5^(n+1)]*(7^n-5^n)/(n*6^n)]
=6*lim{n->inf}(n+1)/n*lim{n->inf}(n+1)/n*lim{n->inf}(7^n-5^n)/[7^(n+1)-5^(n+1)]
=6*1*lim{n->inf}1/7*[1-(5/7)^n]/[1-(5/7)^(n+1)] (分子,分母同除7^(n+1))
=6/7
lim{n->inf}[6^(n+1)*(n+1)/[7^(n+1)-5^(n+1)]*(7^n-5^n)/(n*6^n)]
=6*lim{n->inf}(n+1)/n*lim{n->inf}(n+1)/n*lim{n->inf}(7^n-5^n)/[7^(n+1)-5^(n+1)]
=6*1*lim{n->inf}1/7*[1-(5/7)^n]/[1-(5/7)^(n+1)] (分子,分母同除7^(n+1))
=6/7