Definition:Euclid's Definitions - Book I/21 - Triangles by Angles
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Definition
As Euclid defined it:
- Further, of trilateral figures, a right-angled triangle is that which has a right angle, an obtuse-angled triangle that which has an obtuse angle, and an acute-angled triangle that which has its three angles acute.
(The Elements: Book I: Definition $21$)
