a(n+1)=2an/(an + 2) 1/a(n+1)= (an + 2)/2an = 1/2 + 1/an 1/a(n+1) - 1/an = 1/2 所以1/an 为等差数列,公差是1/2 1/an = 1/1 + 1*(n-1)/2 = (n+1)/2 an = 2/(n+1)
