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Computable Euclid

Proposition 31b

Theorem

The angle (BCE) in a circular segment (BCE) greater than a semicircle is an acute angle.

Commentary

1. Given a circle centered at O, let AB  be a diameter.
2. Let C be any point on the circumference and connect AC  and BC , constructing ACB.
3. Let E be a point on the circumference, so that arc ECB is longer than the semicircle. Connect CE  so that BCE is constructed.
4. Then BCE contained in the segment (the area bounded by arc BCE and BE ) is an acute angle.
5. Book 3 Propositions 31a covers the case when the segment of the circle is equal to a semicircle. Book 3 Propositions 31c covers the case when the segment of the circle is smaller than a semicircle.

Original statement

ἐν κύκλῳ ἡ μὲν ἐν τῷ ἡμικυκλίῳ γωνία ὀρθή ἐστιν, ἡ δὲ ἐν τῷ μϵίζονι τμήματι ἐλάττων ὀρθῆς, ἡ δὲ ἐν τῷ ἐλάττονι τμήματι μϵίζων ὀρθῆς: καὶ ἔτι ἡ μὲν τοῦ μϵίζονος τμήματος γωνία μϵίζων ἐστὶν ὀρθῆς, ἡ δὲ τοῦ ἐλάττονος τμήματος γωνία ἐλάττων ὀρθῆς.

English translation

In a circle the angle in the semicircle is right, that in a greater segment less than a right angle, and that in a less segment greater than a right angle; and further the angle of the greater segment is greater than a right angle, and the angle of the less segment less than a right angle.


Computable version


Additional instances


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