Plane shapes are two-dimensional shapes bounded by lines known as sides. Any shape drawn on a plane is called a two-dimensional shape (or 2-D shapes for short). When we say a figure is two- dimensional, we mean it can be measured along x and y axes i.e. it has length and width or breadth.
Types of Plane Shapes
Common plane shapes are:
- Triangles
- Quadrilaterals
- Polygons
- Circles
Triangles and quadrilaterals are examples of polygon. However, because triangles and quadrilaterals have their own special properties they are usually dealt with separately.
Polygon
A polygon is any closed shape that has three or more straight sides. Thus, rectangles, squares and triangles are all examples of polygons.
The table below shows some special polygons and their sides.
| Names of Polygons | Number of sides | |
| Triangle | 3 | |
| Quadrilateral | 4 | |
| Pentagon | 5 | |
| Hexagon | 6 | |
| Heptagon | 7 | |
| Octagon | 8 | |
| Nonagon | 9 | |
| Decagon | 10 | |
| Duo decagon | 12 |
There are two types of polygons. They are:
- Regular Polygons
- Irregular Polygons
The circumference is the distance around the circle.
A radius (plural radii) is any straight line joining the centre of the circle to any point on the circumference.
A chord is any straight line joining two points on the circumference.
A diameter is any chord that goes through the centre of the circle.
A sectoris the region between two radii and the circumference.
A semicircleis a sector between a diameter and the circumference, i.e. half a circle.
A segment is the region between a chord and the circumference.
Evaluation:
Draw a circle and include the following parts: two radii, a sector, a chord, a segment, a diameter and an arc. Label each part and shade any regions.
Reading Assignment
- Essential Mathematics for J.S.S 1 by A.J.S Oluwasanmi, page 167-169.
- New General Mathematics by M.F Macrae et.al, Revision test II, Numbers 1-9.
Weekend Assignment
- A polygon with seven sides is called ……………… (a) pentagon (b) hexagon (c) octagon (d) heptagon
- The simplest form of polygon is a ………….. (a) circle (b) rectangle (c) triangle (d) square
- Which of the following quadrilaterals has only one pair of parallel sides? (a) Trapezium (b) rhombus (c) parallelogram (d) square
- How many sides has a duo decagon? (a) 10 (b) 20 (c) 12 (d) 9
- A straight line joining two points on the circumference is called …………… (a) chord (b) segment (c) arc (d) sector
Theory
Write down the missing word in the following
- A regular polygon has all its sides …………… and all its angles …………..
- The distance around the circle is ………………………..
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