等差数列{an}的前n项和为Sn,a3+a17=10,则s19是多少

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S19 = a1 + a2 + …… + a19
= (a1 + a19) + (a2+a18) + (a3+a17) + (a4+a16) + …… + (a9+a11) + a10
= 9* (a3+a17) + a10
= 9 * (a3+a17) + (a3+a17)/2
= 9 * 10 + 10/2
= 95
回答中利用了 a1 + a19 = a2 + a18 = a3 + a17 = …… = 2* a10

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