得意小栈
幼苗
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刚才做好的突然没了,重来
Sn=1/6(An+1)(An+2),
a1+a2+a3+...+an=1/6(an+1)(an+2),
6(a1+a2+a3+...+an)=(an+1)(an+2),
6(a1+a2+a3+...+an-1)=a²n-3an+2=(an-1)(an-2),
6(a1+a2+a3+...+an-1)=6Sn-1
6S(n-1)=(a(n-1)+1)(a(n-1)+2)
(a(n-1)+1)(a(n-1)+2)=(an-1)(an-2),
a²(n-1)+3a(n-1)=a²n-3an
a²n-a²(n-1)=3an+3a(n-1)
(an-an-1)(an+an-1)=3(an+an-1)
an-an-1=3.所以an为等差数列
1年前
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