等差数列{an}中,Sn是{an}前n项和,已知S6=2,S9=5,则S15=
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等差数列{an}中,Sn是{an}前n项和,已知S6=2,S9=5,则S15=等差数列{an}中,Sn是{an}前n项和,已知S6=2,S9=5,则S15=等差数列{an}中,Sn是{an}前n项和,
等差数列{an}中,Sn是{an}前n项和,已知S6=2,S9=5,则S15=
等差数列{an}中,Sn是{an}前n项和,已知S6=2,S9=5,则S15=
等差数列{an}中,Sn是{an}前n项和,已知S6=2,S9=5,则S15=
(S9-S6)/3=1=a8=a1+7d,
S6=6*a1+15d=2,
解得a1=-1/27 ,d=4/27
S15=15*a1+91*d=349/27=12+25/27
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