May I ask a question? Is there any difference between the Spivak's Calculus book (the fourth edition) and what is so called (World student series edition)?
What you meant by a dense set is that like a set between, for example, 1 - 100 on the x axis.. Not like the example on that answer whose points on the x axis are smaller than one?
The other answer on my question shows that it's possible on aplane.. He used the fact that there are an infinte numbers of Pythagorean triples.. Take a look on that answer and the comment.. It's so simple
Well, if I want to choose any two points on the unit circle to test if they have a rational distance between each others.. Just give me three or more points to test them..
I see, so in other words, The two points on the unit circle should have a specific angle between each others.. And by moving them we get an infinite set..