A triangle is a closed shape with three straight sides and three angles that always total 180 degrees. Triangles are named two ways at once: by their sides as equilateral, isosceles, or scalene, and by their angles as acute, right, or obtuse.
Math Dictionary / Triangle
Triangle
A triangle is a polygon with three sides and three angles.
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What it means
The angle measures in any triangle add to 180 degrees.
Also called: three-sided polygon
Examples
| A right triangle has one 90-degree angle |
| An equilateral triangle has three equal sides |
| A triangle has 3 vertices |
Common mistake
A triangle must have three straight sides; curved shapes are not triangles.
Try one
Question: What is the sum of the angle measures in a triangle?
Show answer
180 degrees.