因为lim(n->∞)[4^n/(5^n-2^n)]/[4^n/5^n]
=lim(n->∞)[5^n/(5^n-2^n)]
=lim(n->∞)[1/(1-(2/5)^n)]
=1/(1-0)
=1
又因为Σ[4^n/5^n] 是等比级数,且公比|4/5|
因为lim(n->∞)[4^n/(5^n-2^n)]/[4^n/5^n]
=lim(n->∞)[5^n/(5^n-2^n)]
=lim(n->∞)[1/(1-(2/5)^n)]
=1/(1-0)
=1
又因为Σ[4^n/5^n] 是等比级数,且公比|4/5|