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15问答网 > 已知等差数列中｛an｝中，a2+a8=8，则该数列前9项和s9等于？

已知等差数列中｛an｝中，a2+a8=8，则该数列前9项和s9等于？

2025-02-15 10:58:52

推荐回答（2个）

回答1：

已知等差数列{an}中，a2+a8=8，则该数列前9项和s9等于
___36___
因为等差数列{an}中，a1+a9=a2+a8=a3+a7=a4+a6=a5+a5
所以a1+a2+a3+a4+a5+a6+a7+a8+a9=9a5=9(a2+a8)/2
因为数列前9项和s9=a1+a2+a3+a4+a5+a6+a7+a8+a9，a2+a8=8
所以s9=36

回答2：

s9=9（a2+a8）÷2=36

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