Sn=an^2
a1=a1^2
a1=1 或a1=0
S2=a2^2
1+a2=a2^2
(a2-1/2)^2=5/4
a2=1/2+√5/2 或 a2=1/2-√5/2
Sn=an^2
Sn-1=an-1^2
an=Sn-Sn-1=an^2-an-1^2
(an-1/2)^2=(an-1+1/4)
an=1/2+√(an-1+1/4) 或 an=1/2-√(an-1+1/4)
a1=0,a2=a2^2,a2=1或a2=0
因此在沒有更多限制條件時,an很難有統(tǒng)一的通項表示