Definition:Triangle (Geometry)/Obtuse

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An obtuse triangle is a triangle in which one of the vertices is an obtuse angle.

Also known as

An obtuse triangle is also known as an obtuse-angled triangle.

Euclid's 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 $21$)