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Many textbooks define the notion of relation using the notion of n-tuple. So here is my point:

How can you define relation
using n-tuple if an n-tuple itself should be defined
using the notion of ordering (i.e a relation.) ?

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 You usually define $(a,b)$ as a set, $\{a, \{a,b\}\}$. See, e.g., Naive Set Theory, by Halmos. The definition isn't circular. However, this isn't a research question - you should post such questions on math.stackexchange.com. – Benjamin Young Jan 6 2012 at 11:28
ok sorry...In fact I thought math.stackexchange.com had changed its name... – john Jan 6 2012 at 11:33
Duplicate: mathoverflow.net/questions/13031/… – Joel David Hamkins Jan 6 2012 at 12:43

closed as off topic by Emil Jeřábek, Deane Yang, Chandan Singh Dalawat, Tom Leinster, Joel David Hamkins Jan 6 2012 at 12:42

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