Find a term in a sum

### 1.1. Knowledge to remember

6 + 4 = 10 x + 4 = 10 6 + x = 10

6 = 10 – 4 x = 10 – 4 x = 10 – 6

4 = 10 – 6 x = 6 x = 4

So to find a term we subtract the other term from the sum.

### 1.2. Math forms

**Form 1: Find x**

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

**Form 2: Fill in the missing numbers in the table**

– To find the sum, you add the given terms.

– To find the term, you subtract the known term from the sum.

**Form 3: Math Quiz**

– Read and analyze the topic.

– Find the solution of the problem: If you want to find a missing term, part or component, subtract the known terms from the total.

– Present the lesson and check the results just found.

### 1.3. Textbook Exercises Page 45

**Lesson 1**

Find x (by pattern):

a) x + 3 = 9

x = 9 − 3

x = 6

b) x + 5 = 10

c) x + 2 = 8

d) x + 8 = 19

e) 4 + x = 14

g) 3 + x = 10

__Solution method__

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

__Solution guide__

b) x + 5 = 10

x = 10 − 5

x = 5

c) x + 2 = 8

x = 8 − 2

x = 6

d) x + 8 = 19

x = 19 − 8

x = 11

e) 4 + x = 14

x = 14 − 4

x = 10

g) 3 + x = 10

x = 10 − 3

x = 7

**Lesson 2**

Write the correct number in the blank:

__Solution method__

-Find sum: Take two terms and add them together.

– To find a term: Subtract the other term from the sum.

__Solution guide__

lesson 3

A class has 35 students, of which 20 are boys. How many girls are there in that class?

__Solution method__

To find the number of girls, subtract the number of boys in the class from the number of students in the class.

__Solution guide__

The number of girls in that class is:

35 – 20 = 15 (students)

Answer: 15 students.

### 1.4. Solving Textbook Exercises Page 46

Lesson 1

Find x:

a) x + 8 = 10x +8 = 10

b) x + 7 = 10x + 7 = 10

c) 30 + x = 58

__Solution method__

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

__Solution guide__

a) x + 8 = 10

x = 10 − 8

x = 2

b) x + 7 = 10

x = 10 − 7

x = 3

c) 30 + x = 58

x = 58 − 30

x = 28

Lesson 2

Mental arithmetic:

9 + 1 = 8 + 2 = 3 + 7 =

10 – 9 = 10 – 8 = 10 – 3 =

10 – 1 = 10 – 2 = 10 – 7 =

__Solution method__

Perform the calculation and then fill in the blank with the result.

__Solution guide__

9 + 1 = 10 8 + 2 = 10 3 + 7 = 10

10 – 9 = 1 10 – 8 = 2 10 – 3 = 7

10 – 1 = 9 10 – 2 = 8 10 – 7 = 3.

lesson 3

Calculate:

10 – 1 – 2 = 10 – 3 – 4 = 19 – 3 – 5 =

10 – 3 = 10 – 7 = 19- 8 =

__Solution method__

Perform calculations from left to right.

__Solution guide__

10 – 1 – 2 = 7 10 – 3 – 4 = 3 19 – 3 – 5 = 11

10 – 3 = 7 10 – 7 = 3 19 – 8 = 11

Lesson 4

There are 45 oranges and tangerines, including 25 oranges. How many tangerines are there?

__Solution method__

To find the number of tangerines, subtract 45 from the number of oranges.

__Solution guide__

The number of tangerines is:

45 – 25 = 20 (fruit)

Answer: 20 tangerines.

Lesson 5

Circle the letter in front of the correct answer:

Find x, know: x + 5 = 5

A. x = 5

B. x = 10

C. x = 0

__Solution method__

To find a term, we subtract the other term from the sum and then choose the appropriate answer.

__Solution guide__

x + 5 = 5

x = 5 − 5

x = 0

**Circle the answer C**

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