Welcome guest
You're not logged in.
323 users online, thereof 0 logged in

Definition: 1.21: Right Triangle, Obtuse Triangle, Acute Triangle

(Definition 21 from Book 1 of Euclid’s “Elements”)

And further of the trilateral figures: a right-angled triangle is that having a right angle, an obtuse-angled (triangle) that having an obtuse angle, and an acute-angled (triangle) that having three acute angles.

Modern definition

A triangle with:



A right triangle:

An acute triangle:

A obtuse triangle:

| | | | | created: 2017-02-04 15:17:05 | modified: 2019-02-15 07:24:29 | by: bookofproofs | references: [6419]

Edit or AddNotationAxiomatic Method

This work was contributed under CC BY-SA 4.0 by:

This work is a derivative of:

CC BY-SA 4.0

Freely downloadable

[6419] Fitzpatrick, Richard: “Euclid’s Elements of Geometry”,, 2007

Bibliography (further reading)