设Sn是等差数列{an}的前n项和,若S
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设Sn是等差数列{an}的前n项和,若
S
设等差数列首项为a,公差为d,
∴s3=3a+3d s6=6a+15d s7=7a+21d ∵ S3 S6= 1 3, ∴ 3a+3d 6a+15d= 1 3 ∴a=2d ∴ S6 S7= 6a+15d 7a+21d= 27 35 故答案为: 27 35
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设Sn是等差数列{an}的前n项和,若S7=35,则a4=______.
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