等差数列an的前n项和为sn若s6/S3=3,则s9/S6=?
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等差数列an的前n项和为sn若s6/S3=3,则s9/S6=?
s6/S3=3,所以a1=2d s9/s6=3a1+12d/2a1+5d=2
Sn=a1+(n-1)d
所以(a1+5d)/(a1+2d)=3解得d=-2a1
s9/s6=(a1+8d)/(a1+5d)=-15a1/(-3a1)=5
容易知道公差为1/2,S9/S6=2
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