6、等差数列{an}前n项和为Sn,满足S20=S40,则下列结论中正确的是( D )A、S30是Sn中的最大值 B、S30是Sn中的最小值C、S30=0 D、S60=0等差数列的Sn公式图像不是二次函数图像么,图像不就是关于20+40=3
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