113
Area of irregular shapes
Textbook: Part 4, lesson 113, pp. 16–18
GoalArea of irregular shapes: counting squares (grid), splitting a shape into parts.
New words
irregular shape · nostandart shaklarea · yuzwhole square · to‘la katakpart square · chala katak
Explanation
The area of an irregular shape can be found in two ways. 1) With a grid or squared paper: add the whole squares and half the number of part squares; the result is approximate. 2) Split the shape into parts whose areas can be found by formulas (for example rectangles) and add or subtract the areas. If a notebook square is 5 mm × 5 mm, 4 squares make 1 cm².
Worked examples
14 whole and 6 part squares: 14 + 6 : 2 = 17 squares (approximately).
An L shape: a 10 × 6 rectangle with a 4 × 3 corner cut out: 60 − 12 = 48.
Class activity
“Handprint”: children trace a hand on squared paper and count whole and part squares to estimate the area of the hand.
Practice
1
A shape has 14 whole and 6 part squares, each square 1 cm². What is the approximate area in cm²?
17
2
A shape is made of two rectangles: 8 cm × 3 cm and 4 cm × 2 cm. What is its area in cm²?
32
3
A 4 cm × 3 cm rectangle is cut from the corner of a 10 cm × 6 cm rectangle. What is the area of the rest in cm²?
48
4
Notebook squares are 5 mm × 5 mm. A shape covers 36 squares. What is its area in cm²?
9
5
One 5 mm square has area 25 mm². What is the area of 20 squares in mm²?
500