等差数列｛an｝的前n项和为Sn,若S13=65,则a7等于多少

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等差数列｛an｝的前n项和为Sn,若S13=65,则a7等于多少等差数列｛an｝的前n项和为Sn,若S13=65,则a7等于多少等差数列｛an｝的前n项和为Sn,若S13=65,则a7等于多少解因为S

等差数列｛an｝的前n项和为Sn,若S13=65,则a7等于多少
等差数列｛an｝的前n项和为Sn,若S13=65,则a7等于多少

等差数列｛an｝的前n项和为Sn,若S13=65,则a7等于多少
解
因为
S13=a1+a2+...+a13
=(a1+a13)+(a2+a12)+..+(a6+a8)+a7
=13a7
所以 a7=S13/13=65/13=5

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