Math Class 1

# Decimeter

Decimeter this lesson, you will be introduced to the second unit of length, the decimeter. You will repeat what a centimeter is, learn about the relationship between these two units of measurement, learn how to correctly write down a new measurement value. You will have the opportunity to practice comparing different lengths and calculate them.

### Task with measures of different lengths

In this lesson, we will consider the second unit of length, which is called the decimeter.

Let’s take a look at the measurements. The figure shows a segment and three measures of different lengths: red, yellow and green (Fig. 1).

Rice. one

Count how many times each measure fits into the segment (Fig. 2).

Rice. 2

The red measure fits into the segment 2 times, yellow – 3 times, green – 4 times. Why are there different results? We took measures of different lengths.

### Centimeter

In the old days, people faced the same problem when measuring length, so single generally accepted units of length were introduced. What common measure do you already know? Centimeter. With this generally accepted measure, we measure the length of the segment (Fig. 3).

Rice. 3

The measure is stacked six times.

We conclude: the length of the segment is 6 cm.

Using a measure of 1 cm, measure the length of the second segment (Fig. 4).

Rice. four

A measure of 1 cm is too small for large segments. You need to take more. To get it, you need to take a measure of 1 cm and set it aside 10 times (Fig. 5).

Rice. 5

### Decimeter

We set aside 10 cm. People agreed to call this new unit of length decimeter . Decimeter is translated as “a tenth of a meter”. It is written like this – 1 dm . There are ten centimeters in one decimeter  (Fig. 6).

Rice. 6

Using a new measure, measure the length of a large segment (Fig. 7).

Rice. 7

The measure is taken three times.

Conclusion: the length of the segment is 3 dm.

Rice. 7

The measure is taken three times.

### Conclusion: the length of the segment is 3 dm.

How many centimeters is this? Convert decimeters to centimeters.

The number 10 is a dozen units, 1 dm is 10 cm. 3 dm is three tens of centimeters.

3 dm = 10 cm + 10 cm + 10 cm = 30 cm.

So 3 dam = 30 cm.

### Convert centimeters to decimeters

Convert centimeters to decimeters.

70 cm – how many decimeters is it? 70 cm is 7 tens of centimeters.

70 cm = 7 dm.

### The task for determining the length of the segment

Determine the length of the segment and express it in decimeters and centimeters. The length of the segment is determined by the ruler (Fig. 8).

Rice. eight

The length of the segment is 13 cm.

The length of the segment is 13 cm.

13 cm = 1 dm 3 cm.

### Comparison of units of length

Practice comparing the new unit of length.

1. 2 dm and 5 dm;

2 < 5;

2 dm < 5 dm.

2. 1 dm 8 cm and 1 dm 7 cm.

The first parts are the same:

1 dm 8 cm and 1 dm 7 cm;

8 > 7;

1 dm 8 cm > 1 dm 7 cm.

### Addition and subtraction of units of length

Perform addition and subtraction with units of length (Fig. 9).

Rice. 9

The solution of the examples is shown in the figure (Fig. 10).

Rice. ten

### Measurement of long distances

At the beginning of the lesson, you learned that a decimeter is needed to measure large segments. Look at the pictures and determine which objects are more convenient to measure in decimeters (Fig. 11).

Rice. eleven

It is more convenient to measure a bench and boards in decimeters.

Task for the curious: draw 1 dm in a notebook in a cell and count how many cells fit into 1 dm.

### Summing up the lesson

In this lesson, we learned about the second unit of length, which is called the decimeter. We learned that there are 10 centimeters in one decimeter, we learned how to correctly write down a new measurement value, we learned how to compare different lengths and calculate it.

Bibliography

1. Aleksandrova L. A., Mordkovich A. G. Mathematics 1st grade. – M: Mnemosyne, 2012.
2. Bashmakov M. I., Nefedova M. G. Mathematics. 1 class. – M: Astrel, 2012.
3. Bedenko M. V. Mathematics. 1 class. – M7: Russian word, 2012.

1. Internet portal “900igr.net” ( Source )
2. Internet portal “slideshare.net” ( Source )
3. Internet portal “900igr.net” ( Source )

### Quest for the curious

Homework

1. What is a decimeter? How to abbreviate decimeter? What is a decimeter?

2. Determine the length of the segments and express it in decimeters and centimeters

3. Perform addition and subtraction with units of length.

1. 1 dm + 3 dm =
2. 4 dm 3 cm + 2 cm =
3. 6 dm 7 cm – 2 dm =
4. 10 dm 2 cm – 1 cm =

Measurement

Relationship between number and geometry. Part 1. Measurements in geometry. Shape properties

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