Definitions and namesA polygon is simply a closed 2-dimensional shape made up of straight lines. The word itself actually means 'many-sided shape'. Obviously, the polygon with the least number of sides is the triangle. Here are the names of all the polygons with up to 12 sides: TrianglesTriangles can be classified using their sides or their angles. The angles in a triangle always add up to 1800. Special types of triangleEquilateral - All 3 sides equal and all 3 angles equal (to 600). Isosceles - Two sides the same and the angles at the bottom of those sides are equal. Scalene - No sides or angles equal. You can also classify triangles by their angles : Acute-angled - All angles are acute (less than 900). Right-angled - One angle is 900. Obtuse-angled - One angle is obtuse (greater than 900). AreaThe area of a triangle is However, you must use the perpendicular height. That means at 900 to the base.