Bn出错了Bn的通项是1/a(2^n)=1/(a·2^n)=(2-1)/(a·2^n)=2/a[(1/2^n-1/2^(n+1))]Bn=2/a[1/2-1/2^(n+1)]＜2/a·1/2=1/a而当n≥2时，An＞1/a所以An＞Bn
uggu
