39
Estimating area approximately
Textbook: Part 2, lesson 39, pp. 13–14
GoalEstimate the area of a figure on a grid by counting whole, half and quarter squares.
New words
approximate · taqribiygrid square · katakwhole square · butun katakapproximately equal · ≈
Explanation
If the boundary of a figure is curved, its area cannot be found by a formula, so we estimate it. A grid of unit squares is laid over the figure. First count the whole squares that lie completely inside, then combine the pieces of boundary squares by eye to estimate how many whole squares they make. The total is the approximate area of the figure and is written with the sign “≈”. Smaller squares give a more accurate result. In a notebook 4 squares make 1 cm².
Worked examples
A leaf shape has 14 whole squares and the edge pieces together make about 6 squares. Its area is ≈ 14 + 6 = 20 cm².
If a shape covers 36 notebook squares, since 4 squares = 1 cm², its area is ≈ 36 : 4 = 9 cm².
Class activity
“Leaf area”: pupils trace a leaf on squared paper, count whole and part squares and estimate its area in cm² (collect leaves with an adult).
Practice
1
A shape has 14 whole squares and pieces making about 6 squares (1 square = 1 cm²). About how many cm² is its area?
20
2
A shape covers 52 notebook squares. If 4 squares = 1 cm², about how many cm² is its area?
13
3
On a plan a floor covers 48 squares, each 50 cm × 50 cm. 4 squares = 1 m². How many m² is the floor?
12
4
Why is an area found with a grid called approximate?
The edge pieces are judged by eye, so the result is not exact.