解析：[n●4^n/5^n]^(1/n)=[n●(4/5)^n]^(1/n)=n^(1/n)●[(4/5)^n]^(1/n)=n^(1/n)●(4/5)
(n.4^n/5^n)^(1/n)=n^(1/n) . ( 4^n/5^n)^(1/n)=n^(1/n) . ( 4/5)
