A polygon with three sides is called a triangle. All triangles consist of three lines that are not collinear and three angles. Polygons can have any number of sides, but they must be closed figures.
Triangles can be categorized based on the properties of their sides and angles. Equilateral triangles have equal sides and equal angles. Isosceles triangles have two equal sides and two equal angles. Scalene triangles have no equal sides and no equal angles. Triangles can also be categorized based on their inside angle. Right triangles have a 90-degree inside angle. Acute triangles have inside angles of less than 90 degrees, and obtuse triangles have inside angles of greater than 90 degrees.