64
Perimeter of regular shapes
Textbook: Part 2, lesson 64, pp. 78–80
GoalPerimeter of a regular polygon: P = a × n.
New words
regular polygon · muntazam ko‘pburchakequal-sided · teng tomonlinumber of sides · tomonlar sonimultiplication · ko‘paytirish
Explanation
All sides of a regular shape are equal. So we find the perimeter by multiplying the side length by the number of sides: P = a × n. A regular triangle with side 3 cm has perimeter 3 × 3 = 9 cm. Reverse tasks: side a = P : n, number of sides n = P : a.
Worked examples
A regular pentagon with side 6 cm: P = 6 × 5 = 30 cm.
A regular octagon with perimeter 480 mm has side 480 : 8 = 60 mm.
Class activity
“Wire shapes”: groups get equal sticks and build a regular triangle, a square and a pentagon, then calculate the perimeters.
Practice
1
A square has side 14 cm. What is its perimeter in cm?
56
2
A regular decagon (10 sides) has side 18 m. Find its perimeter.
180
3
A regular heptagon (7 sides) has perimeter 84 dm. How long is a side, in dm?
12
4
A regular polygon has side 15 cm and perimeter 90 cm. How many sides does it have?
6
5
A regular hexagon has side 25 cm. How many cm of wire are needed to make it?
150