(1) 令bn=1/a(n-1)an,求数列{bn}的前n项和Tn (其中an=2n-1)(2) 令bn=1/a(n+1)an,求数列{bn}的前n项和Tn,试比较Tn与3/2的大小 (其中an=-2n+5)
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![(1) 令bn=1/a(n-1)an,求数列{bn}的前n项和Tn (其中an=2n-1)(2) 令bn=1/a(n+1)an,求数列{bn}的前n项和Tn,试比较Tn与3/2的大小 (其中an=-2n+5)](/uploads/image/z/14984882-26-2.jpg?t=%EF%BC%881%EF%BC%89+%E4%BB%A4bn%3D1%2Fa%28n-1%29an%2C%E6%B1%82%E6%95%B0%E5%88%97%7Bbn%7D%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8CTn+%EF%BC%88%E5%85%B6%E4%B8%ADan%3D2n-1%EF%BC%89%EF%BC%882%EF%BC%89+%E4%BB%A4bn%3D1%2Fa%28n%2B1%29an%2C%E6%B1%82%E6%95%B0%E5%88%97%7Bbn%7D%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8CTn%2C%E8%AF%95%E6%AF%94%E8%BE%83Tn%E4%B8%8E3%2F2%E7%9A%84%E5%A4%A7%E5%B0%8F+%28%E5%85%B6%E4%B8%ADan%3D-2n%2B5%29)
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(1) 令bn=1/a(n-1)an,求数列{bn}的前n项和Tn (其中an=2n-1)(2) 令bn=1/a(n+1)an,求数列{bn}的前n项和Tn,试比较Tn与3/2的大小 (其中an=-2n+5)
(1) 令bn=1/a(n-1)an,求数列{bn}的前n项和Tn (其中an=2n-1)
(2) 令bn=1/a(n+1)an,求数列{bn}的前n项和Tn,试比较Tn与3/2的大小 (其中an=-2n+5)
(1) 令bn=1/a(n-1)an,求数列{bn}的前n项和Tn (其中an=2n-1)(2) 令bn=1/a(n+1)an,求数列{bn}的前n项和Tn,试比较Tn与3/2的大小 (其中an=-2n+5)
(1)把an=2n-1代入bn=1/a(n-1)an=1/(2(n-1)-1)(2n-1)=1/(2n-3)(2n-1)
Tn=b1+b2+b3....+bn-1+bn=1/(2-3)(2-1)+1/(4-3)(4-1)+1/(6-3)(6-1)+....+1/(2n-3)(2n-1)
=-1+(1/2)(1/(4-3)-1/(4-1)+1/(6-3)-1/(6-1)+....1/(2n-5)-1/(2n-3)+1/(2n-3)-1/(2n-1))
=-1+(1/2)(1-1/(2n-1))=-1+(n-1)/(2n-1)=-n/(2n-1)