111
Perimeter of regular shapes
Textbook: Part 4, lesson 111, pp. 4–6
GoalKnow what a formula is; find the perimeter of a regular polygon and of a square by formula.
New words
formula · formularegular polygon · muntazam ko‘pburchakside · tomonperimeter · perimetr
Explanation
All sides of a regular polygon are equal. The perimeter is the sum of all the side lengths. For a regular polygon with n sides of length a, P = a × n. A square is a regular quadrilateral, so P = a × 4. A short written rule that links quantities is called a formula.
Worked examples
A regular octagon has side 35 cm. P = 35 × 8 = 280 cm.
A square has perimeter 52 dm. Its side is a = P : 4 = 52 : 4 = 13 dm.
Class activity
“Formula master”: the teacher names a shape and its side (for example “regular pentagon, side 6 cm”). Children write the formula and find the perimeter. If you like, bend pieces of wire into the shapes and check the perimeter by measuring.
Practice
1
A regular hexagon has side 9 cm. What is its perimeter in cm?
54
2
A square plot has side 25 m. What is its perimeter in m?
100
3
A flag in the shape of a regular triangle has perimeter 42 cm. What is the length of one side in cm?
14
4
Dilnoza wants to make a frame in the shape of a regular pentagon with side 15 cm. How many cm of wire does she need?
75