an=2^(n-1)/(1+2^(n-1))
用数学归纳法证明
ak=2^(k-1)/(1+2^(k-1))
ak+1=2ak/(1+ak)
=2/(1+1/a^k)
=2/(1+(1+2^(k-1))/2^(k-1))
=2^k/(1+2^k)