已知等差数列前三项依次为a,4,3a,前n项的和为Sn,且Sk=110.(1)求a的值和该数列的前n项和公式Sn.(2)求k的值.在考试
人气:297 ℃ 时间:2020-04-03 12:29:47
解答
(1)因为等差数列前三项依次为a,4,3a,所以2*4=a+3a,a=2,前三项则为2,4,6.
d=4-2=2,a1=2,所以Sn=[2a1+(n-1)d] n/2=[4+2n-2]*n/2=n(n+1)
(2)因为Sk=110,所以k(k+1)=110
则k=10或-11
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