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Euclid · Elements §11.prop.20-11.prop.21

Relations and Sum of Plane Angles Forming Solid Angles

Passage 250 of 316 · Greek

Summary

In Book 11, Proposition 20, it is proved that for any solid angle contained by three plane angles, the sum of any two angles is greater than the remaining one. In Proposition 21, it is shown that the sum of all the plane angles containing any solid angle is less than four right angles.

§11.prop.20ἐὰν στερεὰ γωνία ὑπὸ τριῶν γωνιῶν ἐπιπέδων περιέχηται, δύο ὁποιαιοῦν τῆς λοιπῆς μείζονές εἰσι πάντῃ μεταλαμβανόμεναι. στερεὰ γὰρ γωνία ἡ πρὸς τῷ Α ὑπὸ τριῶν γωνιῶν ἐπιπέδων τῶν ὑπὸ ΒΑΓ, ΓΑΔ, ΔΑΒ περιεχέσθω· λέγω, ὅτι τῶν ὑπὸ ΒΑΓ, ΓΑΔ, ΔΑΒ γωνιῶν δύο ὁποιαιοῦν τῆς λοιπῆς μείζονές εἰσι πάντῃ μεταλαμβανόμεναι.
\nIf a solid angle be contained by three plane angles, any two, taken together in any way, are greater than the remaining one.\nFor let the solid angle at A be contained by three plane angles BAC, CAD, DAB; I say that, of the angles BAC, CAD, DAB, any two, taken together in any way, are greater than the remaining one.
εἰ μὲν οὖν αἱ ὑπὸ ΒΑΓ, ΓΑΔ, ΔΑΒ γωνίαι ἴσαι ἀλλήλαις εἰσίν, φανερόν, ὅτι δύο ὁποιαιοῦν τῆς λοιπῆς μείζονές εἰσιν.
\nIf then the angles BAC, CAD, DAB are equal to one another, it is manifest that any two are greater than the remaining one.
εἰ δὲ οὔ, ἔστω μείζων ἡ ὑπὸ ΒΑΓ, καὶ συνεστάτω πρὸς τῇ ΑΒ εὐθείᾳ καὶ τῷ πρὸς αὐτῇ σημείῳ τῷ Α τῇ ὑπὸ ΔΑΒ γωνίᾳ ἐν τῷ διὰ τῶν ΒΑΓ ἐπιπέδῳ ἴση ἡ ὑπὸ ΒΑΕ, καὶ κείσθω τῇ ΑΔ ἴση ἡ ΑΕ, καὶ διὰ τοῦ Ε σημείου διαχθεῖσα ἡ ΒΕΓ τεμνέτω τὰς ΑΒ, ΑΓ εὐθείας κατὰ τὰ Β, Γ σημεῖα, καὶ ἐπεζεύχθωσαν αἱ ΔΒ, ΔΓ. καὶ ἐπεὶ ἴση ἐστὶν ἡ ΔΑ τῇ ΑΕ, κοινὴ δὲ ἡ ΑΒ, δύο δυσὶν ἴσαι·
But if not, let the angle BAC be greater, and let there be constructed, on the straight line AB and at the point A on it, in the plane through BAC, the angle BAE equal to the angle DAB, and let AE be made equal to AD, and let the straight line BEC, drawn through the point E, cut the straight lines AB, AC at the points B, C, and let DB, DC be joined.
καὶ γωνία ἡ ὑπὸ ΔΑΒ γωνίᾳ τῇ ὑπὸ ΒΑΕ ἴση· βάσις ἄρα ἡ ΔΒ βάσει τῇ ΒΕ ἐστιν ἴση.
And since DA is equal to AE, and AB is common, two sides are equal to two sides; and the angle DAB is equal to the angle BAE; therefore the base DB is equal to the base BE.
καὶ ἐπεὶ δύο αἱ ΒΔ, ΔΓ τῆς ΒΓ μείζονές εἰσιν, ὧν ἡ ΔΒ τῇ ΒΕ ἐδείχθη ἴση, λοιπὴ ἄρα ἡ ΔΓ λοιπῆς τῆς ΕΓ μείζων ἐστίν.
And since the two sides BD, DC are greater than BC, of which DB was shown equal to BE, therefore the remaining DC is greater than the remaining EC.
καὶ ἐπεὶ ἴση ἐστὶν ἡ ΔΑ τῇ ΑΕ, κοινὴ δὲ ἡ ΑΓ, καὶ βάσις ἡ ΔΓ βάσεως τῆς ΕΓ μείζων ἐστίν, γωνία ἄρα ἡ ὑπὸ ΔΑΓ γωνίας τῆς ὑπὸ ΕΑΓ μείζων ἐστίν.
And since DA is equal to AE, and AC is common, and the base DC is greater than the base EC, therefore the angle DAC is greater than the angle EAC.
ἐδείχθη δὲ καὶ ἡ ὑπὸ ΔΑΒ τῇ ὑπὸ ΒΑΕ ἴση· αἱ ἄρα ὑπὸ ΔΑΒ, ΔΑΓ τῆς ὑπὸ ΒΑΓ μείζονές εἰσιν.
And the angle DAB was also shown equal to the angle BAE; therefore the angles DAB, DAC are greater than the angle BAC.
ὁμοίως δὴ δείξομεν, ὅτι καὶ αἱ λοιπαὶ σύνδυο λαμβανόμεναι τῆς λοιπῆς μείζονές εἰσιν. ἐὰν ἄρα στερεὰ γωνία ὑπὸ τριῶν γωνιῶν ἐπιπέδων περιέχηται, δύο ὁποιαιοῦν τῆς λοιπῆς μείζονές εἰσι πάντῃ μεταλαμβανόμεναι· ὅπερ ἔδει δεῖξαι. §11.prop.21ἅπασα στερεὰ γωνία ὑπὸ ἐλασσόνων τεσσάρων ὀρθῶν γωνιῶν ἐπιπέδων περιέχεται. ἔστω στερεὰ γωνία ἡ πρὸς τῷ Α περιεχομένη ὑπὸ ἐπιπέδων γωνιῶν τῶν ὑπὸ ΒΑΓ, ΓΑΔ, ΔΑΒ· λέγω, ὅτι αἱ ὑπὸ ΒΑΓ, ΓΑΔ, ΔΑΒ τεσσάρων ὀρθῶν ἐλάσσονές εἰσιν.
Similarly indeed we shall show that also the remaining angles, taken two and two together, are greater than the remaining one.\nTherefore, if a solid angle be contained by three plane angles, any two, taken together in any way, are greater than the remaining one; which was to be proved.\n\nEvery solid angle is contained by plane angles which are together less than four right angles.\nLet the solid angle at A be contained by the plane angles BAC, CAD, DAB; I say that the angles BAC, CAD, DAB are together less than four right angles.\nFor let there be taken on each of the straight lines AB, AC, AD any points B, C, D, and let BC, CD, DB be joined.
εἰλήφθω γὰρ ἐφʼ ἑκάστης τῶν ΑΒ, ΑΓ, ΑΔ τυχόντα σημεῖα τὰ Β, Γ, Δ, καὶ ἐπεζεύχθωσαν αἱ ΒΓ, ΓΔ, ΔΒ. καὶ ἐπεὶ στερεὰ γωνία ἡ πρὸς τῷ Β ὑπὸ τριῶν γωνιῶν ἐπιπέδων περιέχεται τῶν ὑπὸ ΓΒΑ, ΑΒΔ, ΓΒΔ, δύο ὁποιαιοῦν τῆς λοιπῆς μείζονές εἰσιν· αἱ ἄρα ὑπὸ ΓΒΑ, ΑΒΔ τῆς ὑπὸ ΓΒΔ μείζονές εἰσιν.
And since the solid angle at B is contained by three plane angles CBA, ABD, CBD, any two are greater than the remaining one; therefore the angles CBA, ABD are greater than the angle CBD.
διὰ τὰ αὐτὰ δὴ καὶ αἱ μὲν ὑπὸ ΒΓΑ, ΑΓΔ τῆς ὑπὸ ΒΓΔ μείζονές εἰσιν, αἱ δὲ ὑπὸ ΓΔΑ, ΑΔΒ τῆς ὑπὸ ΓΔΒ μείζονές εἰσιν· αἱ ἓξ ἄρα γωνίαι αἱ ὑπὸ ΓΒΑ, ΑΒΔ, ΒΓΑ, ΑΓΔ, ΓΔΑ, ΑΔΒ τριῶν τῶν ὑπὸ ΓΒΔ, ΒΓΔ, ΓΔΒ μείζονές εἰσιν.
For the same reasons indeed the angles BCA, ACD are also greater than the angle BCD, and the angles CDA, ADB are greater than the angle CDB; therefore the six angles CBA, ABD, BCA, ACD, CDA, ADB are greater than the three angles CBD, BCD, CDB.
ἀλλὰ αἱ τρεῖς αἱ ὑπὸ ΓΒΔ, ΒΔΓ, ΒΓΔ δυσὶν ὀρθαῖς ἴσαι εἰσίν· αἱ ἓξ ἄρα αἱ ὑπὸ ΓΒΑ, ΑΒΔ, ΒΓΑ, ΑΓΔ, ΓΔΑ, ΑΔΒ δύο ὀρθῶν μείζονές εἰσιν.
But the three angles CBD, BDC, BCD are equal to two right angles; therefore the six angles CBA, ABD, BCA, ACD, CDA, ADB are greater than two right angles.
καὶ ἐπεὶ ἑκάστου τῶν ΑΒΓ, ΑΓΔ, ΑΔΒ τριγώνων αἱ τρεῖς γωνίαι δυσὶν ὀρθαῖς ἴσαι εἰσίν, αἱ ἄρα τῶν τριῶν τριγώνων ἐννέα γωνίαι αἱ ὑπὸ ΓΒΑ, ΑΓΒ, ΒΑΓ, ΑΓΔ, ΓΔΑ, ΓΑΔ, ΑΔΒ, ΔΒΑ, ΒΑΔ ἓξ ὀρθαῖς ἴσαι εἰσίν, ὧν αἱ ὑπὸ ΑΒΓ, ΒΓΑ, ΑΓΔ, ΓΔΑ, ΑΔΒ, ΔΒΑ ἓξ γωνίαι δύο ὀρθῶν εἰσι μείζονες· λοιπαὶ ἄρα αἱ ὑπὸ ΒΑΓ, ΓΑΔ, ΔΑΒ τρεῖς περιέχουσαι τὴν στερεὰν γωνίαν τεσσάρων ὀρθῶν ἐλάσσονές εἰσιν. ἅπασα ἄρα στερεὰ γωνία ὑπὸ ἐλασσόνων τεσσάρων ὀρθῶν γωνιῶν ἐπιπέδων περιέχεται· ὅπερ ἔδει δεῖξαι.
And since the three angles of each of the triangles ABC, ACD, ADB are equal to two right angles, therefore the nine angles of the three triangles, namely CBA, ACB, BAC, ACD, CDA, CAD, ADB, DBA, BAD, are equal to six right angles, of which the six angles ABC, BCA, ACD, CDA, ADB, DBA are greater than two right angles; therefore the remaining three angles BAC, CAD, DAB containing the solid angle are less than four right angles.\nTherefore, every solid angle is contained by plane angles which are together less than four right angles; which was to be proved.

Notes

  1. 11.prop.20πάντῃ μεταλαμβανόμεναι — The present passive participle `μεταλαμβανόμεναι` (from `μεταλαμβάνω`, "to take in exchange or turn") combined with the adverb `πάντῃ` ("in every way") means "taken together in any way" or "in whatever combination they are taken."
  2. 11.prop.20ἐν τῷ διὰ τῶν ΒΑΓ ἐπιπέδῳ — Meaning "in the plane through B, A, C." The word for "points" (likely `σημείων`) is omitted after `διὰ τῶν`, referring to the unique plane determined by the three points B, A, and C (i.e., plane BAC).
  3. 11.prop.20δύο δυσὶν ἴσαι — Meaning "two [sides] are equal to two [sides]." This is a formulaic expression in geometric proofs where the noun "sides" (`πλευραί`) is omitted, indicating that two sides of one triangle are respectively equal to two sides of another.
  4. 11.prop.21ὑπὸ ἐλασσόνων τεσσάρων ὀρθῶν γωνιῶν — Literally "by plane angles less than four right angles." This does not mean that each individual angle is less than four right angles, but rather that the "sum" of all the plane angles containing the solid angle is less than four right angles (360 degrees).

Cite this passage

Euclid, Elements §11.prop.20-11.prop.21. Humanitext Reader, https://reader.humanitext.ai/en/text/urn:cts:greekLit:tlg1799.tlg001.humanitext-grc2:11.prop.20-11.prop.21

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