答:CA(n+1)=2/An +1A(n+2)=2/A(n+1) +1A(n+2)=2/ [2/An +1] +1A(n+2)=2An /(2+An) +1=1+2(2+An -2) /(2+An)A(n+2)=3- 4/(2+An)A1=1,A3=3-4/3=5/3A5=3-4/(2+5/3)=3-12/11=21/11选择C