142
Year review: geometry (perimeter, area) and time
Textbook: Part 4, lesson 142, pp. 97–98
GoalYear review: perimeter and area of rectangles and squares, composite shapes, calculating with time.
New words
perimeter · perimetrarea · yuzside · tomontime interval · vaqt oralig‘i
Explanation
The perimeter of a rectangle is P = (a + b) × 2 and its area is S = a × b. The perimeter of a square is P = a × 4 and its area is S = a × a. Perimeter is measured in units of length (m, cm) and area in square units (m², cm²). We find the area of a composite shape by splitting it into parts and adding, or by subtracting the extra part. To find a time interval we work with hours and minutes separately.
Worked examples
A rectangle is 15 cm long and its width is 3 times smaller, 15 : 3 = 5 cm. P = (15 + 5) × 2 = 40 cm and S = 15 × 5 = 75 cm².
A square has perimeter 28 cm. Its side is 28 : 4 = 7 cm and its area is 7 × 7 = 49 cm². Time: 2 h 40 min + 1 h 35 min = 3 h 75 min = 4 h 15 min.
Class activity
“Garden design”: groups draw a rectangular garden on squared paper and calculate its perimeter (fence length) and area (planting area). We compare who gets the largest area with the same perimeter.
Practice
1
A field is 15 m long and 8 m wide. What is its area in m²?
120
2
A garden is 25 m long and 10 m wide. How many metres of fence are needed to enclose it?
70
3
A square has perimeter 36 cm. What is its area in cm²?
81
4
A rectangle 3 cm by 2 cm is cut out of a rectangle 7 cm by 4 cm. What is the area of the remaining shape in cm²?
22
5
A film began at 16:40 and ended at 18:30. How many minutes did it last?
110