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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:

Notes

Examples

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]

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

Bibliography (further reading)

(none)