2 11 3
26
等差数列计算公式:
项数=(末项-首项)÷公差+1
数列为奇数项时,前n项的和=中间项×项数
数列为偶数项,求首尾项相加,用它的和除以2
等差数列公式:末项=首项+(项数-1)*公差
前n项和公式为:Sn=na1+n(n-1)d/2
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