Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Topic: Induction proof
Replies: 24   Last Post: Aug 22, 2006 4:11 PM

 Messages: [ Previous | Next ]
 Brian M. Scott Posts: 1,289 Registered: 12/6/04
Re: Induction proof
Posted: Aug 20, 2006 2:12 PM

On 20 Aug 2006 10:29:37 -0700, <emailtgs@gmail.com> wrote in

> okay, my question is, just how do you decide to go from here ... to
> here.

> < (1 - 1/n) + 1/(n+1)^2 (by induction hypotheses)
> < (1 - 1/n) + 1/(n*(n+1))

> I know you've made a note of what you did but I don't
> understand how you can do that.

The 'how' is trivial. 1/(n+1)^2 < 1/(n*(n+1)), so of course
(1 - 1/n) + 1/(n+1)^2 < (1 - 1/n) + 1/(n*(n+1)): you're just
adding 1 - 1/n to both sides.

To see why 1/(n+1)^2 < 1/(n*(n+1)), if it isn't already
obvious, observe that (n+1)^2 > n*(n+1). In even more
detail: n+1 > n, so multiplying both sides by n+1 yields
(n+1)^2 > n*(n+1) if n+1 > 0, i.e., if n > -1. Here you
know that n > 1, so certainly n > -1.

The 'why' is so that you can split 1/(n*(n+1)) into partial
fractions, one of which is 1/n that cancels the 1/n that you

[...]

Brian

Date Subject Author
8/17/06 emailtgs@gmail.com
8/17/06 Lynn Kurtz
8/17/06 Lynn Kurtz
8/17/06 emailtgs@gmail.com
8/17/06 emailtgs@gmail.com
8/17/06 Ben Young
8/17/06 emailtgs@gmail.com
8/18/06 Paul Sperry
8/18/06 emailtgs@gmail.com
8/20/06 Brian M. Scott
8/20/06 emailtgs@gmail.com
8/20/06 Brian M. Scott
8/18/06 Torsten Hennig
8/18/06 emailtgs@gmail.com
8/18/06 Torsten Hennig
8/19/06 emailtgs@gmail.com
8/19/06 mon
8/19/06 Dave L. Renfro
8/20/06 mon
8/20/06 emailtgs@gmail.com
8/20/06 Brian M. Scott
8/19/06 Alexander Bogomolny
8/19/06 Alexander Bogomolny
8/19/06 Dave L. Renfro
8/22/06 Alexander Bogomolny