设等差数列{an}的前n项的和为Sn,a1=15,S4=S12,求Sn的最大值.
设等差数列{an}的前n项的和为Sn,a1=15,S4=S12,求Sn的最大值.
数学人气:930 ℃时间:2020-05-14 05:09:20
优质解答
na1+d*n(n-1)/2,等差数列前n项和公式,s4=s12,4a1+6d=12a1+66d,a1=15代入可以求出d,再求出通项公式,继续求出前n项和公式,运用前无古人后无来者就是Sn>Sn-1,Sn<Sn+1,这样解出n就行了,我马上下线,时间不多这就是思路
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