利用等比数列求和公式1+2+4+...+2^n=2^0+2^1+...+2^n=2^0(1-2^(n+1))/(1-2)=2^(n+1) -1
lim(n→∞) (1+2+4+8＋...+2^n)/2^n
= lim(n→∞)((2^(n+1) -1)/2^n)
= lim(n→∞)(2- 1/2^n)
=2-0
=2