Obtuse Triangle

A triangle where one of the internal angles is obtuse (greater than 90 degrees).

Try this Drag the orange dots on any vertex to reshape the triangle. While any angle exceeds 90 degrees, it is an obtuse triangle.

In any triangle, two of the interior angles are always acute (less than 90 degrees)^*, so there are three possibilities for the third angle:

Less than 90° - all three angles are acute and so the triangle is acute.
Exactly 90° - it is a right triangle
Greater than 90° (obtuse): the triangle is an obtuse triangle

In the figure above, drag the vertices around and try to create all 3 possibilities. The title will change to reflect the type of triangle you have created. (You may have to move the mouse slowly to get an angle to be exactly 90°).

* To see why this is so: The internal angles of any triangle always add up to 180°. If two angles were greater than 90° they would add to more then 180° just by themselves. Therefore this can never happen.
(Prove it to yourself - reshape the triangle above and try to get two angles to be greater than 90°).

Obtuse Triangle

Other triangle topics

General

Perimeter / Area

Triangle types

Triangle centers

Congruence and Similarity

Solving triangles

Triangle quizzes and exercises