117
Perimeter of a polygon
Textbook Part 4, pp. 14–15
GoalKnow that perimeter is the sum of all side lengths, and calculate it.
New words
perimeter · perimetrregular polygon · muntazam ko‘pburchakequal sides · teng tomonlar
Explanation
The sum of the lengths of all sides of a polygon is its perimeter. A polygon with all sides equal is a regular polygon. A rectangle with sides 6 cm and 3 cm: P = 6 + 3 + 6 + 3 = 18 cm.
Worked examples
Triangle: 3 cm, 4 cm, 5 cm → P = 12 cm.
A doll box is a 20 cm square: P = 20 + 20 + 20 + 20 = 80 cm.
Class activity
Children run a string all round the edge of a desk and measure the string — that is the desk’s perimeter. Then they measure each side and add to check.
Practice
1
A square has 4 cm sides. Perimeter?
16 cm
2
A triangle has sides 2 cm, 5 cm, 6 cm. Perimeter?
13 cm
3
Write in decimetres: 30 cm, 50 cm
3 dm, 5 dm
4
Which is not regular: a square or a 6 by 3 rectangle?
The rectangle