And further of the trilateral figures: a

right-angled triangleis that having a right angle, anobtuse-angled(triangle) that having an obtuse angle, and anacute-angled(triangle) that having three acute angles.

A triangle with:

- a right angle is called
**right-angled**(or**right**), - one obtuse angle is called
**obtuse-angled**(or**obtuse**), - three acute angles is called
**acute-angled**(or**acute**).

- Euclid implicitly renames the term “3-sided figure” to the term “triangle”, and defines special kinds of triangles, without defining the general term “triangle”. However, this term is used frequently in Euclid’s “Elements” in its general meaning.

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]

[6419] **Fitzpatrick, Richard**: “Euclid’s Elements of Geometry”, http://farside.ph.utexas.edu/Books/Euclid/Euclid.html, 2007

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