83
Finding area with a grid (practice)
Textbook: Part 3, lesson 83, pp. 48–50
GoalPractise finding area with a grid; the area, unknown side and perimeter of a rectangle.
New words
rectangle · to‘g‘ri to‘rtburchaklength · bo‘yiwidth · enipart · bo‘lak
Explanation
Take squares of the grid or notebook as the unit of area: 1 square = 1 cm². The area of a rectangle = length × width. If the area and one side are known, the other side = area : known side. If a line cuts a rectangle into two parts, the areas of the parts add up to the whole area. Perimeter = (length + width) × 2.
Worked examples
If the area is 840 cm² and the length 42 cm, the width is 840 : 42 = 20 cm; the perimeter is (42 + 20) × 2 = 124 cm.
An 8 cm × 5 cm rectangle (40 cm²) is cut into parts of 12 cm² and 28 cm²: 12 + 28 = 40.
Class activity
“Find the area”: groups draw a rectangle on squared paper and find its area in two ways (counting squares and multiplying) and compare.
Practice
1
A rectangle has area 360 cm² and width 12 cm. How many centimetres long is it?
30
2
A rectangle has area 540 cm² and length 27 cm. What is its perimeter in centimetres?
94
3
A 15 cm × 8 cm rectangle is cut into two parts. The first part has area 45 cm². What is the area of the second part in cm²?
75
4
A shape has 18 whole and 8 part squares, one square is 1 cm². About how many cm² is its area?
22
5
A veranda floor needs 280 yellow tiles, 90 fewer green tiles than that, and 50 more red tiles than green ones. How many tiles are needed in total?
710