由S10=100,S20-10 代入等差求和公式Sn=nA1+[n(n-1)D]/2中
可以得到
10A1+[10(10-1)D]/2 =100.公式1
20A1+[20(20-1)D]/2 =10.公式2
可求得第一个数字为A1=18.55
公差为D=-1.9
则S30=30A1+[30(30-1)D]/2=30*18.55+(29*30*D/2)=-270
Sn是等差数列{an}的前n项和,已知S10=100,S20=10,求S30.
Sn是等差数列{an}的前n项和,已知S10=100,S20=10,求S30.
数学人气:901 ℃时间:2019-09-09 11:34:26
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