设等差数列平{an}的前n项和为Sn,若a1=-11,a4+a8=-6.则当Sn取最小值时,n等于?
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![设等差数列平{an}的前n项和为Sn,若a1=-11,a4+a8=-6.则当Sn取最小值时,n等于?](/uploads/image/z/2718732-12-2.jpg?t=%E8%AE%BE%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%E5%B9%B3%7Ban%7D%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B8%BASn%2C%E8%8B%A5a1%3D-11%2Ca4%2Ba8%3D-6.%E5%88%99%E5%BD%93Sn%E5%8F%96%E6%9C%80%E5%B0%8F%E5%80%BC%E6%97%B6%2Cn%E7%AD%89%E4%BA%8E%3F)
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设等差数列平{an}的前n项和为Sn,若a1=-11,a4+a8=-6.则当Sn取最小值时,n等于?
设等差数列平{an}的前n项和为Sn,若a1=-11,a4+a8=-6.则当Sn取最小值时,n等于?
设等差数列平{an}的前n项和为Sn,若a1=-11,a4+a8=-6.则当Sn取最小值时,n等于?
a4+a8=2a1+10d=2×(-11)+10d=-22+10d=-6
10d=16
d=8/5
an=a1+(n-1)d=-11+(8/5)(n-1)=(8n-63)/5
令an<0
(8n-63)/5<0
n<63/8,又n为正整数,n≤7,即数列前7项均<0,从第8项开始,以后各项均为正.前7项和最小.当Sn取最小值时,n=7.
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