2 INTRODUCTION theorems and must be proved by a particular line of reasoning. To produce this reasoning, we impose on ourselves the conditions of the hypothesis and, assuming that these conditions are satisfied, we must deduce the facts stated in the conclusion. According to this, we allow that a certain fact is true: 1◦. If it is part of the hypothesis 2◦. If it is part of the definition of one of the elements under discussion 7 3◦. If it follows from an axiom 4◦. If it follows from an earlier proof. No fact can be considered certain, in a geometric argument, if it does not follow from one of the preceding four reasons. 2b. A proposition whose conclusion is formed, completely or partially, by the hypothesis of a first proposition, and whose hypothesis is formed, completely or partially, by the conclusion of the first proposition, is called a converse of the first proposition. An immediate consequence of a theorem is called a corollary. On the other hand, a lemma is a preparatory proposition, intended to facilitate the proof of a later proposition. 3. Congruent figures. An arbitrary geometric figure can be moved through space without deformation in infinitely many ways, as is the case with ordinary solid objects. Two figures are called congruent if one can be moved onto the other, in such a way that all their parts coincide 8 in other words, two congruent figures are one and the same figure, in two different places. A figure which is subject to displacement without deformation is also called an invariant figure. 4. The Straight Line. The simplest of all lines is a straight line, of which a stretched thread gives us an idea. The notion of a straight line is clear by itself in order to use it in our reasoning, we consider the straight line to be defined by its obvious properties, in particular the following: 1◦. Any figure congruent to a straight line is itself a straight line and con- versely, every straight line can be made to coincide with any other straight line, and this may be done so that an arbitrarily chosen point on the first coincides with any point chosen on the second 2◦. Through any two points we can draw one, and only one, straight line. Thus we can talk about the straight line passing through points A and B or, more briefly, of the straight line AB. It follows immediately from the definition that two different lines can meet in only one point, since, if they had two common points, they could not be distinct. 7 It often happens in the course of a proof that one introduces auxiliary elements into the figure. A fact can then be true by virtue of the definition of these new elements. We then say that the fact is true by construction. 8 Hadamard uses the term equal throughout to describe figures that can be made to coincide. The term congruent is used more often in English. We will use either term for line segments and angles (but most often the term equal for these simple figures), and the term congruent for other, more complicated figures.–transl.

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