两边同乘2an 2anSn=an²+1
2(Sn-Sn-1)Sn=(Sn-Sn-1)²+1
(Sn-Sn-1)【2Sn-(Sn-Sn-1)】=1
Sn²-Sn-1²=1 a1=Sn=1
Sn²=n
an=Sn-Sn-1=√n-√(n-1)