Math 2 Lesson: Subtraction Table

### 1.1. Knowledge to remember

– Table subtraction within 20

– Use the addition and subtraction table within 20 to do consecutive addition and subtraction.

– How to use the subtraction table within 20 to calculate mentally, except remember within 100 to solve the problem about less.

– How to find subtracted numbers, unknown terms.

### 1.2. Math forms

**Form 1: Mental calculation**

– Review the learned calculations.

– Mental subtraction can be based on how to calculate addition.

**Form 2: Perform consecutive addition and subtraction operations.**

– Expressions containing only addition or subtraction operations, you perform them in turn order from left to right.

**Form 3: Find the missing element**

To find the unknown term, subtract the known term from the sum.

– To find the number to be subtracted unknown, we take the difference plus the minus number.

### 1.3. Solving Textbook Exercises page 69

**Lesson 1**

Mental arithmetic:

11 – 2 = 12 – 3 = 13 – 4 =

11 – 3 = 12 – 4 = 13 – 5 =

11 – 4 = 12 – 5 = 13 – 6 =

11 – 5 = 12 – 6 = 13 – 7 =

11 – 6 = 12 – 7 = 13 – 8 =

11 – 7 = 12 – 8 = 13 – 9 =

11 – 8 = 12 – 9 =

11 – 9 =

14 – 5 = 15 – 6 = 16 – 7 = 17 – 8 =

14 – 6 = 15 – 7 = 16 – 8 = 17 – 9 =

14 – 7 = 15 – 8 = 16 – 9 =

14 – 8 = 15 – 9 = 18 – 9 =

14 – 9 =

__Solution method__

Subtract the numbers and then fill in the blanks with the results.

__Solution guide__

11 – 2 = 9 12 – 3 = 9 13 – 4 = 9

11 – 3 = 8 12 – 4 = 8 13 – 5 = 8

11 – 4 = 7 12 – 5 = 7 13 – 6 = 7

11 – 5 = 6 12 – 6 = 6 13 – 7 = 6

11 – 6 = 5 12 – 7 = 5 13 – 8 = 5

11 – 7 = 4 12 – 8 = 4 13 – 9 = 4

11 – 8 = 3 12 – 9 = 3

11 – 9 = 2

14 – 5 = 9 15 – 6 = 9 16 – 7 = 9 17 – 8 = 9

14 – 6 = 8 15 – 7 = 8 16 – 8 = 8 17 – 9 = 8

14 – 7 = 7 15 – 8 = 7 16 – 9 = 7

14 – 8 = 6 15 – 9 = 6 18 – 9 = 9

14 – 9 = 5

**Lesson 2**

Calculate:

5 + 6 – 8 = 9 + 8 – 9 = 3 + 9 – 6 =

8 + 4 – 5 = 6 + 9 – 8 = 7 + 7 – 9 =

__Solution method__

Perform calculations in order from right to left.

__Solution guide__

5 + 6 – 8 = 3 9 + 8 – 9 = 8 3 + 9 – 6 = 6

8 + 4 – 5 = 7 6 + 9 – 8 = 7 7 + 7 – 9 = 5

### 1.4. Solving Textbook Exercises page 70

**Lesson 1**

Mental arithmetic:

18 – 9 = 16 – 8 = 14 – 7 = 17 – 9 =

17 – 8 = 15 – 7 = 13 – 6 = 12 – 8 =

16 – 7 = 14 – 6 = 12 – 5 = 16 – 6 =

15 – 6 = 13 – 5 = 11 – 4 = 14 – 5 =

12 – 3 = 12 – 4 = 10 – 3 = 11 – 3 =

__Solution method__

Mentally calculate and then fill in the blank with the result of the calculation.

__Solution guide__

18 – 9 = 9 16 – 8 = 8 14 – 7 = 7 17 – 9 = 8

17 – 8 = 9 15 – 7 = 8 13 – 6 = 7 12 – 8 = 4

16 – 7 = 9 14 – 6 = 8 12 – 5 = 7 16 – 6 = 10

15 – 6 = 9 13 – 5 = 8 11 – 4 = 7 14 – 5 = 9

12 – 3 = 9 12 – 4 = 8 10 – 3 = 7 11 – 3 = 8

**Lesson 2**

Calculate then calculate:

a) 35 – 8; 57 – 9; 63 – 5;

b) 72 – 34; 81 – 45; 94 – 36.

__Solution method__

– Set calculation: Write the calculation so that the digits are in the same row and column together.

– Calculation: Perform subtraction in order from right to left.

__Solution guide__

**lesson 3**

Find x:

a) x + 7 = 21

b) 8 + x = 42

c) x – 15 = 15.

__Solution method__

– To find a term, we subtract the other term from the sum.

– To find the number to be subtracted we take the difference of addition and subtraction.

__Solution guide__

a) x + 7 = 21

x = 21 – 7

x = 14

b) 8 + x = 42

x = 42 – 8

x = 34

c) x – 15 = 15

x = 15 + 15

x = 30

**Lesson 4**

The big box has 45 kg of sugar, the small one has 6 kg less sugar than the big one. How many kilograms of sugar in the baby box?

__Solution method__

To find how many kilograms of sugar are in the small box, we subtract 6 kg from the mass of the big box.

__Solution guide__

The baby box has the number of kilograms of sugar:

45 – 6 = 39 (kg)

Answer: 39 kg of sugar.

**Lesson 5**

Choose a letter that has a right answer:

How many centimeters long is the line MN?

A. About 7cm

B. About 8cm

C. About 9cm

D. About 10cm.

__Solution method__

– Compare the line segment MN with the given length 1dm and then estimate the length of the line segment MN.

– Choose the best answer.

__Solution guide__

Line segment MN is slightly shorter than line segment 1dm.

That 1dm = 10cm, so the length of the line segment MN is about 9cm.

**Circle the answer C**

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