已知等差数列｛An}的前n项和记为Sn,a5=15,a10=25 1.通项An 2.若Sn=112,求n
人气：363 ℃ 时间：2019-11-05 04:03:31
解答
1、a₅ = a₁ ＋4d ＝15a₁₀ = a₁ ＋9d ＝25则 a₁ ＝7 ,d＝2即通项An＝7＋2(n-1)=2n+52、Sn ＝(a₁+an)n/2 = (12+2n)n/2 = n^2 + 6n =112即n^2 + 6n －112 ＝0(n -14)(n+8)=0...
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