问题: 简单数学题
The first three terms of an arithmetic progressin are 5,9,13
Find
a)The common difference of the progression
b)The sum of the first 20 terms after the 3rd term.
解答:
解：
(a)The common difference d=4
(2)S23=23(a1+a23)/2=23(2a1+22d)/2=23(2×5+22×4)/2=1127
S3=5+9+13=27
The sum of the first 20 terms after the 3rd term=1127-27=1100.
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