# Definition:Euclid's Definitions - Book I/21 - Triangles by Angles

Jump to navigation
Jump to search

## Definition

In the words of Euclid:

*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 $\text{I}$: Definition $21$)

## Sources

- 1926: Sir Thomas L. Heath:
*Euclid: The Thirteen Books of The Elements: Volume 1*(2nd ed.) ... (previous) ... (next): Book $\text{I}$. Definitions