数列{an}满足a1=1,an+1*根号(1/an^2+4)=1,S2n+1-Sn

1个回答

  • ∵数列{a[n]}满足a[n+1]√(1/a[n]^2+4)=1

    ∴1/a[n+1]^2-1/a[n]^2=4

    ∵a[1]=1

    ∴{1/a[n]^2}是首项为1/a[1]^2=1,公差为4的等差数列

    即:1/a[n]^2=1+4(n-1)=4n-3

    ∴a[n]^2=1/(4n-3)

    ∵S[n]=a[1]^2+a[2]^2+……+a[n]^2

    【问题补充时,楼主将第一个“+”号写成“=”,应该是“笔”误,这里改过来.】

    ∴(S[2n+1]-S[n])-(S[2n+3]-S[n+1])

    =(a[n+1]^2+a[n+2]^2+...+a[2n+1]^2)-(a[n+2]^2+a[n+3]^2+...+a[2n+1]^2+a[2n+2]^2+a[2n+3]^2)

    =a[n+1]^2-a[2n+2]^2-a[2n+3]^2

    =1/(4n+1)-1/(8n+5)-1/(8n+9)

    ∵1/(8n+2)>1/(8n+5),1/(8n+2)>1/(8n+9),1/(4n+1)=1/(8n+2)+1/(8n+2)

    ∴(S[2n+1]-S[n])-(S[2n+3]-S[n+1])>0

    即:S[2n+1]-S[n]>S[2n+3]-S[n+1]

    说明{S[2n+1]-S[n]}是一个递减数列

    ∴{S[2n+1]-S[n]}最大项为:S[3]-S[1]=a[2]^2+a[3]^3=1/5+1/9=14/45

    ∵S[2n+1]-S[n]≤m/30对n属于N*恒成立

    ∴S[2n+1]-S[n]≤S[3]-S[1]≤m/30

    即:14/45≤m/30

    解得:m≥28/3

    ∴m的最小值是28/3