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数学
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设Sn是等差数列{an}的前n项和,a12=-8,S9=-9,证明数列{Sn/n}是等差数列
人气:224 ℃ 时间:2020-06-28 04:01:27
解答
a12=a1+11d=-8 s9=(a1+a9)*9/2=9a1+36d=-9
所以 a1=3 d=-1
sn/n=a1+(n-1)d/2=3+(1-n)/2 =7/2-n*1/2
所以是等差数列
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