等差数列、等比数列前n项和公式???

Question

如题所述

Answer 1

等差：Sn=a1n+(n-1)nd/2=(a1+an)n/2
等比：Sn=a1(1-q^n)/(1-q)=(a1-anq)/(1-q) (q不等于1)
Sn=na1(q=1)

Answer 2

sn=n(a1+an)/2=na1+n(n-1)d/2
sn=a1(1-q^n)/(1-q)

但要特别注意n=1要单独考虑。
看是否a1=s1，若不相等则应分n=1，n大于等于2来写

Answer 3

sn=n(a1+an)/2=na1+n(n-1)d/2
an=a1(1-q^n)/(1-q)本回答被提问者采纳

Answer 4

等差数列前N项和公式S=（a1+an)*n/2

Answer 5

等差数列 Sn=na1+n(n-1)d/2=(a1+an)*n/2
等比数列 q=1 an=a1 ,q不为1时 an=a1*q^（n－1)