98
The 2 times table
Textbook: Part 3, lesson 92, pp. 44–45
GoalUse multiplication by 2; the rectangle perimeter formula P = (a + b) × 2.
New words
a pair · juftlikpupil on duty · navbatchiperimeter · perimetrmultiplication · ko‘paytirish
Explanation
Opposite sides of a rectangle are equal, so 10 + 10 + 6 + 6 can be written as 10 × 2 + 6 × 2. The perimeter formula is P = a × 2 + b × 2 or P = (a + b) × 2. If 2 pupils are on duty each day, 15 pairs are 15 × 2 = 30 pupils.
Worked examples
With sides 4 cm and 3 cm: P = (4 + 3) × 2 = 14 cm, S = 4 × 3 = 12 cm².
2 posters for each of 28 classrooms: 28 × 2 = 56 posters.
Class activity
“Twice over”: children draw a rectangle and find its perimeter two ways: by adding all sides and with (a + b) × 2. Do the results match?
Practice
1
A school has 28 classrooms with 2 posters in each. How many posters in total?
56
2
A teacher formed 15 pairs of duty pupils. How many pupils are on duty?
30
3
A rectangle is 9 cm long and 5 cm wide. What is its perimeter in cm?
28
4
A rectangle with sides 6 cm and 2 cm: what is its area in cm²?
12
5
A rectangle is 10 cm by 6 cm. What is its perimeter P = 10 × 2 + 6 × 2 in cm?
32