1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Proof for inequality induction

  1. Oct 17, 2007 #1
    proof for inequality induction...please help!!

    1. The problem statement, all variables and given/known data

    Prove for all positive integers n that
    [tex]\sum^{n}_{l=1} l^{-1/2} > 2(\sqrt{n+1} -1)[/tex]

    2. The attempt at a solution
    [tex]\sum^{n+1}_{l=1} l^{-1/2}
    = \sum^{n}_{l=1} l^{-1/2} + (n+1)^{-1/2} > 2(\sqrt{n+1} -1) + (n+1)^{-1/2} [/tex]

    please help meee! i'm getting stuck, on how to express [tex] 2(\sqrt{n+1} -1) + (n+1)^{-1/2} [/tex] in terms of [tex] 2(\sqrt{n+2} -1) [/tex]
    Last edited: Oct 17, 2007
  2. jcsd
  3. Oct 17, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    Are you sure you are supposed to prove this by induction? I'm having a hard time doing it that way. On the other hand there is an easy way to do it by looking at an upper Riemann sum and comparing it with the integral of x^(-1/2).
  4. Oct 17, 2007 #3
    yes im pretty sureee..because that's what im learning right nowww :frown:
  5. Oct 17, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper

    Ok, so what you want to do is now show that 2*(sqrt(n+1)-1)+1/sqrt(n+1)>2*(sqrt(n+2)-1). Right? Multiply both sides by sqrt(n+1). The simple sqrt(n+1) radicals cancel on both sides. The sqrt(n+1)sqrt(n+2) remains on the right. Square both sides. It just barely works.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Proof for inequality induction