Polygon with three sides and three vertices.

Triangles can be classified by the lengths of their sides:

• Equilateral triangle: all the sides have the same length
• Isosceles triangle: two sides have the same length
• Scalene triangle: all the sides have different length.

Triangles can also be classified by their internal angles:

• Acute triangle: all the internal angles measured less than $90\degree$
• Right(-angled) triangle: one of its internal angles measures $90\degree$
• Obtuse triangle: one of its internal angle measures more than $90\degree$

The sum of the internal angles of a triangle is $180\degree$

The area of a triangle is given by the formula:

where $b$ is the base and $h$ the height of the triangle.