设Sn为等差数列{an}的前n项和,S8=4a3,a7=-2,a9=?
1个回答
a1+a8=a2+a7=a3+a6=a4+a5
故S8=4(a3+a6)=4a3+4a6
而又有S8=4a3
故a6=0
而a7=-2
故d=-2
故a9=a7+2d=-6
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