n（n+1）/2=[n(n+1)(n+2)-(n-1)n(n+1)]/6
1+3+6+…＋ n（n+1）/2
=[(6-0)+(24-6)+(60-24)+...+n(n+1)(n+2)-(n-1)n(n+1)]/6
=n(n+1)(n+2)/6
an=n(n+1)/2
2Sn=1^2+2^2+3^2+……+n^2 +1+2+3+……+n +n*1
=(n+1)(2n+1)/6 +(1+n)n/2 +n
Sn易求
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