Math 2 Lesson: Decimeter

### 1.1. Knowledge to remember

The decimeter is a unit of length. Decimeter is abbreviated as dm.

1dm = 10cm

10cm = 1dm

### 1.2. Math forms

**Form 1: Compare the lengths of the line segments**

– Measure the length of the straight lines (if necessary)

– Compare and use “shorter” phrases; “longer”; “smaller”; “greater than”… to conclude.

**Form 2: Convert measurement units.**

Remember: 1dm = 10cm, 10cm = 1dm

**Form 3: Perform calculations with units of length.**

– Convert numbers to the same unit of measure (form 2)

– Perform math operations with numbers and keep the unit in the result.

### 1.3. Solving Textbook Exercises Page 7

**Lesson 1**

Look at the picture and answer the questions:

a) Which dot is less than or greater than?

– Length of line segment AB ………… 1dm.

– Length of line segment CD ………… 1dm.

b) Which dot is shorter or longer?

– Line segment AB ………… line segment CD.

– Line segment CD ………… line segment AB.

__Solution method__

– Observe the image, the length of the line segment is 1dm and the length of the segments AB, CD.

– Choose the appropriate phrase and fill in the blanks.

__Solution guide__

a)

– Length of line segment AB is greater than 1dm

– The length of the line segment CD is less than 1dm

b)

– Line segment AB is longer than line segment CD

– Line segment CD is shorter than line AB

**Lesson 2**

Calculation (according to the form):

a)

1dm + 1dm = 2dm

3dm + 2dm =

8dm + 2dm =

9dm + 10dm =

b)

8dm – 2dm = 6dm

16dm – 2dm =

10dm – 9dm =

35dm – 3dm =

__Solution method__

Add or subtract the numbers and then write the length unit after the result you just found.

__Solution guide__

a)

1dm + 1dm = 2dm

3dm + 2dm = 5dm

8dm + 2dm = 10dm

9dm + 10dm = 19dm

b)

8dm – 2dm = 6dm

16dm – 2dm = 14dm

10dm – 9dm = 1dm

35dm – 3dm = 32dm

Without using a ruler, estimate the length of each line segment and then write the appropriate number in the dot:

__Solution method__

– Estimate by eye, then fill in the blank with the appropriate number.

Note: 1dm = 10cm.

__Solution guide__

### 1.4. Solving Textbook Exercises page 8

**Lesson 1**

a) Number?

10cm = … dm; 1dm = … cm

b) Find on a straightedge the line indicates 1dm.

c) Draw a line segment AB 1dm long.

__Solution method__

Apply knowledge of 1dm = 10cm to solve the questions.

__Solution guide__

a) 10cm = 1dm; 1dm = 10cm

b) On the ruler, the 10cm line is the 1dm line.

c) Students draw a 1dm line by themselves (drawing from the 0cm line to the 10cm line)

a) Find on a straightedge the line indicates 2dm.

b) Number ? 2dm = … cm

__Solution method__

Apply knowledge of 1dm = 10cm to solve the questions.

__Solution guide__

a) The 20cm line on the ruler is the 2dm line.

b) 2dm = 20cm.

**lesson 3**

a) 1dm = … cm 3dm = … cm 8dm = … cm

2dm = … cm 5dm = … cm 9dm = … cm

b) 30cm = … dm 60cm = … dm 70cm = … dm

__Solution method__

Apply knowledge of 1dm = 10cm and then fill in the blanks with the appropriate number.

__Solution guide__

a) 1dm = 10cm 3dm = 30cm 8dm = 80cm

2dm = 20cm 5dm = 50cm 9dm = 90cm

b) 30cm = 3dm 60cm = 6dm 70cm = 7dm

**Lesson 4**

Enter cm or dm in the appropriate dot:

__Solution method__

Estimate the lengths of the distances, compare them with the given numbers, and then fill in the blanks with appropriate units.

Note: 1dm = 10cm.

__Solution guide__

The length of the pencils is 16cm

– The length of the mother’s hand is 2dm

– Khoa’s step length is 30cm

– Baby Phuong is 12dm tall

.

=====

## Leave a Reply