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Computable Euclid

Proposition 8a

Theorem

Alternate name(s): peacock tail.

Let P be any point outside a circle and let A, B, C, D, E, and F be points on the circumference of the circle. If PA  is the extension of PD  and passes through the center O of the circle, PB  is the extension of PE , PC  is the extension of PF , and PB  is nearer to PA  than PC  is to PA , then PA  > PB  > PC , and PD  < PE  < PF .

Commentary

1. Given a circle centered at O, let P be any point outside the circle.
2. Construct a line through P and O. PO  intersects the circle at D on the convex circumference and at A on the concave circumference.
3. Find two points E and F on the convex circumference and construct line segments PE  and PF , such that PE  is nearer to PD  than PF  is to PD . (While Euclid did not define "nearer" in this context, his proof suggests he had comparison of angles in mind, so that we are assuming DPE < ∠DPF.) Extend PE  and PF  so that they intersect the concave circumference at B and C, respectively.
4. Then PA  > PB  > PC , and PD  < PE  < PF .
5. Book 3 Propositions 8b and 8c deal with the question of when line segments from P to the circumference can be equal in length.
6. Book 3 Proposition 7a is similar except P is inside of the circle.

Original statement

ἐὰν κύκλου ληϕθῇ τι σημϵῖον ἐκτός, ἀπὸ δὲ τοῦ σημϵίου πρὸς τὸν κύκλον διαχθῶσιν ϵὐθϵῖαί τινϵς, ὧν μία μὲν διὰ τοῦ κέντρου, αἱ δὲ λοιπαί, ὡς ἔτυχϵν, τῶν μὲν πρὸς τὴν κοίλην πϵριϕέρϵιαν προσπιπτουσῶν ϵὐθϵιῶν μϵγίστη μέν ἐστιν ἡ διὰ τοῦ κέντρου, τῶν δὲ ἄλλων ἀϵὶ ἡ ἔγγιον τῆς διὰ τοῦ κέντρου τῆς ἀπώτϵρον μϵίζων ἐστίν, τῶν δὲ πρὸς τὴν κυρτὴν πϵριϕέρϵιαν προσπιπτουσῶν ϵὐθϵιῶν ἐλαχίστη μέν ἐστιν ἡ μϵταξὺ τοῦ τϵ σημϵίου καὶ τῆς διαμέτρου, τῶν δὲ ἄλλων ἀϵὶ ἡ ἔγγιον τῆς ἐλαχίστης τῆς ἀπώτϵρόν ἐστιν ἐλάττων, δύο δὲ μόνον ἴσαι ἀπὸ τοῦ σημϵίου προσπϵσοῦνται πρὸς τὸν κύκλον ἐϕ᾽ ἑκάτϵρα τῆς ἐλαχίστης.

English translation

If a point is taken outside a circle and from the point straight lines are drawn through to the circle, one of which is through the centre and the others are drawn at random, then, of the straight lines which fall on the concave circumference, that through the centre is greatest, while of the rest the nearer to that through the centre is always greater than the more remote, but, of the straight lines falling on the convex circumference, that between the point and the diameter is least, while of the rest the nearer to the least is always less than the more remote, and only two equal straight lines will fall on the circle from the point, one on each side of the least.


Computable version


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