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17 Give an example to show why the least upper bound axiom close not applay to the set of rational numbers?
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Answer #1

We know that Least Upper bound axiom means a ordered set S in wich for every nonempty subset A which is bounded above then supA i.e least upper bound of A exists in  set S.

But Q not satisfy this axiom.

Because observe the following subset

A={x€Q /x²<2}

Then clearly A is nonempty bounded subset of Q

Because it is bounded by all elements x€Q and x²>2

Now we observe that SupA=√2= square root of 2

But we know that √2=square root of 2 does not belong to Q

Since it is one irrational number

So clearly supA does belong to Q

I. E Q has no least upper bound property

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