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等差数列{An},6S5-5S3=5,则A4=
等差数列{An},6S5-5S3=5,则A4=
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2020-03-10 23:24
等差数列{An},6S5-5S3=5,则A4=
等差数列{An},6S5-5S3=5,则A4=
∵6S5-5S3=5
∴6×[a1+a1+(5-1)d]/2-5×[a1+a1+(3-1)d]/2=5
(30a1+60d)-(15a1+15d)=5
15a1+45d=5
3a1+9d=1
∴a1+3d=1/3=A4