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Congruence as explained by Apostol in Calculus Volume 1.

  1. Apr 3, 2012 #1
    1) I am having trouble understanding some parts of congruence as is explained by T Apostol in the book Calculus volume 1.
    In the chapter on integration , while discussing area functions he explains congruence as follows :-
    Now I can appreciate that concepts such as 'area' , 'congruence' are being defined in terms of sets. But , the definition of congruence makes use of the notion of 'distance' . And while calculating distance between two points one has to make use of the Pythagoras theorem. And as far as I know any proof of Pythagoras theorem makes use of congruence of triangles in one way or the other. I don't know whether I am being fussy here but aren't we having circularity of reasoning here. Or are there proofs of Pythagoras theorem not depending on congruence of triangles (or for that matter notions of distance not making use of Pythagoras theorem).

    2) Furthermore , he defines rectangles as those sets of points which are congruent to a set of the form
    S = {(x,y)| 0≤x≤h , 0≤y≤k}
    where h and k are lengths of the edges of the rectangle.
    Later by an axiom , area of a rectangle is taken as hk.
    My question is , is the area 'hk' according to the axiom* only for the set S ? If so,do we have to show that more general rectangles are 'congruent' to the set S , by making use of 'congruence' as defined in terms of sets. (by general rectangles I mean rectangles which are rotated and/or translated versions of the rectangle represented by set S).

    And do we have to do this also for other geometric objects such as triangles?

    * axiom 5 - choice of scale - Every rectangle is measurable . If a rectangle has lengths of edges h and k then its area is hk.

    Overall , I am having some trouble in seeing the motivation behind defining congruence and rectangles as is done in the book.
    Last edited: Apr 3, 2012
  2. jcsd
  3. Apr 3, 2012 #2
  4. Apr 3, 2012 #3
    Euclid's proof makes use of "congruent triangles"
    the above quote is from 7) in the link to Euclid's proof.

    Other proofs that I have seen use similar triangles and so on. All these notions are not defined by Apostol in his book.
    One proof I saw makes use of differential calculus but that concept is taken up in chapters after integration.

    Perhaps , the notion of Euclidean distance(or the Pythagorean theorem) is taken as an undefined concept in Apostol's book. And the concept of 'congruence' derived using that.
    After all , we are free to choose the 'undefined' objects and their axioms as long as they are consistent. My query is whether Apostol has done so with respect to the Pythagorean theorem.
  5. Apr 3, 2012 #4


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    You're worried far too late in the game. Apostol assumes Pythagoras is true, so you don't have to worry about consistency at that point (it's not a geometry textbook). Your question should be: how does congruence of triangles make sense in the proof of Pythagoras, independent of what Apostol says congruence means? (He's surely allowed to re-define the term given that it's a different subject of mathematics)

    In Euclid, triangles are congruent if all their side and angle lengths are the same (it should be clear that Apostol's definition gives this condition for triangles, but note that this does not depend on Apostol's definition at all). This doesn't require any notion of Pythagoras's theorem
  6. Apr 3, 2012 #5
    Thanks. This helped. And yes , the re-definition was satisfactory as it was in terms of sets.
    But I was having trouble seeing the motivation behind it.I only understood that he is expressing congruence in terms of sets of real numbers.

    Could you also explain to me the query I raised in point 2) in post 1. (I hope I am making sense :shy:)
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