等差数列重要公式
奇数项与偶数项的关系
差系
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\(\text{if} \; N = 2n, S_{奇} - S_{偶} = -nd;\)
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\(\text{if} \; N = 2n + 1, S_{奇} - S_{偶} = a_{n + 1}(中间项).\)
比系
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\(\text{if} \; N = 2n, \dfrac{S_{奇}}{S_{偶}} = \dfrac{a_n}{a_{n + 1}};\)
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\(\text{if} \; N = 2n + 1, \dfrac{S_{奇}}{S_{偶}} = \dfrac{n + 1}{n};\)