等差数列{an},a1=-3,11a5=5a8,其前n项和为Sn,求Sn最小值
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等差数列{an},a1=-3,11a5=5a8,其前n项和为Sn,求Sn最小值
等差数列{an},a1=-3,11a5=5a8,其前n项和为Sn,求Sn最小值
等差数列{an},a1=-3,11a5=5a8,其前n项和为Sn,求Sn最小值
设等差数列的公差为d,则由11a5=5a8得
11(a1+4d)=5(a1+7d);a1=-3
得d=2
则an=a1+(n-1)d=-3+(n-1)*2=2n-5
sn=n(a1+an)/2=n(-3+2n-5)/2=n^2-4n =(n-2)^-4
Sn的最小值=-4
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