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Bonjourno, I'm trying to work at the lectures provided on youtube by nptelhrd but I've gotten my foot stuck in a hole in the real line only 15 minutres into it

(In my head I say "such that" whenever the symbol : pops up!).

We define a set;

A: {r ∈Q: r²<2}

Does ∃ a largest element ofAinQ?

1:We seek to find some n ∈N: [tex] ( r \ + \ \frac{1}{n} ) [/tex] will satisfy the conditions specified byA.

2:[tex] (r \ + \ \frac{1}{n} )^2 \ < \ 2 [/tex]

3:[tex] r^2 \ + \ \frac{2r}{n} \ + \ \frac{1}{n^2} \ < \ 2 [/tex]

4:[tex] \frac{2r}{n} \ + \ \frac{1}{n^2} \ < \ 2 \ - \ r^2 [/tex]

The R.H.S. is strictly positive due to r²<2.

Okay, I understand up to here but then the lecturer starts to get confusing, he then says thatIt suffices only to find somen ∈N:

[tex] \frac{2r}{n} \ + \ \frac{1}{n} \ < \ 2 \ - \ r^2 [/tex]

Notice the n and not n² on the bottom of the L.H.S. Fraction!

He says;

This is because;

[tex] \frac{1}{n^2} < \frac{1}{n} \ and \ this \ implies \ \frac{2r}{n} \ + \ \frac{1}{n^2} \ < \ \frac{2r}{n} \ + \ \frac{1}{n} [/itex]

I have no idea where this came from!

The video is on youtube http://www.youtube.com/watch?v=0lzO...DB30C539B&playnext_from=PL&index=0&playnext=1 and I would say everything he is trying to do is described from 10:00 to 14:00.

I wouldextremelyappreciate it if someone could take 6 minutes to watch this and correct me as I have nobody else to explain it to me.

What I think is going on is that he is trying to prove a least upper bound or something and that this will show that the real line can be continuously divided, or something.

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# Defining Elements on the Real Line?

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