Triangles have three sides. There are many different types of triangles:
3 equal sides
3 equal angles of 60°
2 equal sides
2 equal angles
No equal sides
No equal angles
One of its angles is a right angle (90°)
Quadrilaterals have four sides. Here are some special quadrilaterals:
4 equal sides
4 right angles
2 pairs of equal sides
4 right angles
4 equal sides
Opposite sides are parallel
Opposite angles are equal
2 pairs of equal sides
Opposite sides are parallel
Opposite angles are equal
One pair of parallel sides of different lengths
2 pairs of equal sides next to each other
No parallel sides.
All circles have a circumference, diameter and radius:
The circumference is the distance all the way around a circle.
The diameter is the distance right across the middle of the circle.
The radius is the distance halfway across the circle. The radius is always half the length of the diameter.
Polygons are shapes with many straight sides:
Regular polygons have equal angles and sides of equal length.
Irregular polygons have sides of different lengths.
Here are some common polygons:
Pentagons have 5 sides:
Hexagons have 6 sides:
Heptagons have 7 sides:
Octagons have 8 sides:
The perimeter is the distance all the way around the outside of a 2D shape.
To work out the perimeter, add up the lengths of all the sides.
The perimeter of this shape is 5 + 5 + 10 + 10 = 30 cm:
The area of a 2D shape is the amount of surface it covers.
The units for area are cm^{2} (square centimetres), m^{2} (square metres) or km^{2} (square kilometres).
To work out the area of a rectangle, multiply its length (the longer side) by its width (the shorter side):
area = length × width
The area of this rectangle is 6 x 4 = 24 cm^{2}: