Proposition I.4

Triangles with Equal Sides have equal angles

If two triangles have the two sides equal to two sides respectively, and have the angles contained by the equal straight lines equal, they will also have the base 1 equal to the base, the triangle will be equal to the triangle, and the remaining angles 2 will be equal to the remaining angles respectively, 3 namely those which the equal sides subtend. 4

Let ABC, DEF be two triangles having the two sides AB, AC equal to the two sides DE, DF respectively, namely AB to DE and AC to DF, and the angle BAC 5 equal to the angle EDF.

I say that the base BC is also equal to the base EF, the triangle ABC will be equal to the triangle DEF, and the remaining angles will be equal to the remaining angles respectively, namely those which the equal sides subtend, that is, the angle ABC to the angle DEF, and the angle ACB to the angle DFE.

For, if the triangle ABC be applied to the triangle DEF,

  • and if the point A be placed on the point D and the straight line AB on DE,

then the point B will also coincide 6 with E, because AB is equal to DE.

Again, AB coinciding with DE, the straight line AC will also coincide with DF, because the angle BAC is equal to the angle EDF;

  • hence the point C will also coincide with the point F, because AC is again equal to DF.

But B also coincided with E; hence the base BC will coincide with the base EF.

[For if, when B coincides with E and C with F, the base BC does not coincide with the base EF, two straight lines will enclose a space: which is impossible.

  • Therefore the base BC will coincide with EF] 7 and will be equal to it. I.c.n.4

Thus the whole triangle ABC will coincide with the whole triangle DEF,

  • and will be equal to it.

And the remaining angles will also coincide with the remaining angles and will be equal to them, the angle ABC to the angle DEF, and the angle ACB to the angle DFE.

Therefore etc. 8

  • (Being) what it was required to prove.

References

graph BT I_4[ I.4 ]:::prop; click I_4 "/elem.1.4" "Book I Proposition 4"; %%%% dependencies I_cn4( I.cn.4 ):::cn; click I_cn4 "/elem.1.c.n.4" "Book I Common Notion 4"; %%%% links I_4 --> I_cn4

Footnotes

  1. the base.
    Here we have the word base used for the first time in the Elements. Proclus explains it (p. 236, 12-15) as meaning (1), when no side of a triangle has been mentioned before, the side which is on a level with the sight (τὴν πρὸς τῇ ὄψει κειμένην), and (2), when two sides have already been mentioned, the third side. Proclus thus avoids the mistake made by some modern editors who explain the term exclusively with reference to the case where two sides have been mentioned before. That this is an error is proved (1) by the occurrence of the term in the enunciations of I. 37 etc. about triangles on the same base and equal bases, (2) by the application of the same term to the bases of parallelograms in I. 35 etc. The truth is that the use of the term must have been suggested by the practice of drawing the particular side horizontally, as it were, and the rest of the figure above it. The base of a figure was therefore spoken of, primarily, in the same sense as the base of anything else, e.g. of a pedestal or column; but when, as in I. 5, two triangles were compared occupying other than the normal positions which gave rise to the name, and when two sides had been previously mentioned, the base was, as Proclus says, necessarily the third side. ↩

  2. enunciation
    It is a fact that Euclid's enunciations not infrequently leave something to be desired in point of clearness and precision. Here he speaks of the triangles having the angle equal to the angle, namely the angle contained by the equal straight lines (τὴν γωνίαν τῇ γωνίᾳ ἴσην ἔχῃ τὴν ὑπὸ τῶν ἴσων εὐθειῶν περιεχομένην), only one of the two angles being described in the latter expression (in the accusative), and a similar expression in the dative being left to be understood of the other angle. It is curious too that, after mentioning two sides, he speaks of the angles contained by the equal straight lines, not sides.
    It may be that he wished to adhere scrupulously, at the outset, to the phraseology of the definitions, where the angle is the inclination to one another of two lines or straight lines. Similarly in the enunciation of [I. 5] he speaks of producing the equal straight lines as if to keep strictly to the wording of [Postulate 2. ↩

  3. respectively.
    I agree with Mr H. M. Taylor (Euclid, p. ix) that it is best to abandon the traditional translation of each to each,
    which would naturally seem to imply that all the four magnitudes are equal rather than (as the Greek ὲκατέρα ὲκατέρᾳ does) that one is equal to one and the other to the other. ↩

  4. subtend.

    ὑποτείνειν ὑπό

    ↩

  5. the angle BAC.
    The full Greek expression would be ἡ ὑπὸ τῶν ΒΑ, ΑΓ περιεχομένη γωνία, the angle contained by the (straight lines) BA, AC.
    But it was a common practice of Greek geometers, e.g. of Archimedes and Apollonius (and Euclid too in Books X.—XIII.), to use the abbreviation αἱ ΒΑΓ for αἱ ΒΑ, ΑΓ, the (straight lines) BA, AC.
    Thus, on περιεχομένη being dropped, the expression would become first ἡ ὑπὸ τῶν ΒΑΓ γωνία, then ἡ ὑπὸ ΒΑΓ γωνία, and finally ἡ ὑπὸ ΒΑΓ, without γωνία, as we regularly find it in Euclid. ↩

  6. if the triangle be applied to...coincide.
    The difference between the technical use of the passive ἐφαρμόζεσθαι to be applied (to), and of the active ἐφαρμόζειν to coincide (with)
    has been noticed above (note on Common Notion 4, pp. 224-5). ↩

  7. [For if, when B coincides...36. coincide with EF].
    Heiberg (Paralipomena su Euklid in Hermes, XXXVIII., 1903, p. 56) has pointed out, as a conclusive reason for regarding these words as an early interpolation, that the text of an-Nairīzī (Codex Leidensis 399, 1, ed. Besthorn-Heiberg, p. 55) does not give the words in this place but after the conclusion Q.E.D., which shows that they constitute a scholium only. They were doubtless added by some commentator who thought it necessary to explain the immediate inference that, since B coincides with E and C with F, the straight line BC coincides with the straight line EF, an inference which really follows from the definition of a straight line and Post. 1; and no doubt the Postulate that Two straight lines cannot enclose a space (afterwards placed among the Common Notions) was interpolated at the same time. ↩

  8. Therefore etc.
    Where (as here) Euclid's conclusion merely repeats the enunciation word for word, I shall avoid the repetition and write Therefore etc.
    simply. ↩