A polygon with three internal angles that add up to 180° and has at least two equal sides is called what?

A polygon with three internal angles that add up to 180° is defined as a triangle since this is a fundamental property of triangles in Euclidean geometry. Among the types of triangles, the specific term for a triangle that has at least two sides of equal length is an isosceles triangle. This type of triangle not only has the property of two equal sides but also has the angles opposite those equal sides being equal, reinforcing the definition.

The correct identification of isosceles triangles is crucial in understanding the classification of triangles based on their side lengths and angle measures, which is a key concept in geometry. Other classifications like scalene triangles have no equal sides, equilateral triangles have all sides equal, and right triangles possess a 90-degree angle, none of which specifically denote the property of having at least two equal sides.