等差数列

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等差数列等差数列 等差数列an=3n-1,a1=3-1=2,所以Sn=(a1+an)n/2=(3n-1+2)n/2=n(3n+1)/2全部展开收起(3n+1)n/2a1=3*1-1=2a(n

等差数列
等差数列
 

等差数列
an=3n-1,a1=3-1=2,
所以Sn=(a1+an)n/2=(3n-1+2)n/2=n(3n+1)/2

收起

(3n+1)n/2

a1=3*1-1=2
a(n-1)=3(n-1)-1
d=an-a(n-1)=3
Sn=na1+n(n-1)d/2=((3n+n)/2

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