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