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Homework Help: Help with induction?

  1. Sep 27, 2009 #1
    Sorry if this is a really stupid question :( I'm a bit confused on something. I have the whole proof written out and I've checked it everywhere, but I'm confused on WHY for one step.

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

    2. Relevant equations

    3. The attempt at a solution

    This is for proving 1^2+2^2+...+n^2 = n(n+1)(2n+1) / 6. So, you add (n+1)^2 and all that. I

    http://img199.imageshack.us/img199/6412/mathwtf.jpg [Broken]
    http://img199.imageshack.us/img199/6412/mathwtf.jpg [Broken]

    From the second to the third line...I understand most of the rearrangement...but where did the square go??? Why does it go frmo ending with 6(n+1)^2 to ending with 6(n+1)? Where is the square??

    Thank you. Also, any tips for induction? I get other problems, but sometimes it is not clear to me at all how I'm supposed to manipulate it to equal what I need it to equal. Any general tips? Sometimes I try to work backwards for inspiration
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 27, 2009 #2


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    Homework Helper

    In the third line, after the 6(n+1) put another closing bracket ) and then you will see that they just simply factored out an 'n+1' from the n(2n+1)(n+1) and 6(n+1)2.

    Well sometimes you do indeed need to work backwards a bit. And in your example you know in induction your final answer for the sum of (N+1) squares would be just replacing the 'n' (in the inductive hypothesis) by 'N+1'

    example, say I want to show that 1+2+3+...+n = n(n+1)/2

    If I am proving this by induction, I know that my final thing should be

    1+2+3+..+N+(N+1) = [N+1]([N+1]+1)/2
  4. Sep 27, 2009 #3
    OHHHHHHHHHH!!! I see it now! Thank you so much!

    And thanks for the tip, I'll definitely keep that in mind =)

    Again, thank you so much for sorting out my slow little brain, lol.
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