I. Key points of knowledge 1

1, commonly used length units: meters and centimeters.

2. To know the length of an object, you can measure it in feet.

3. The measurement unit of shorter objects is usually centimeters, and the measurement unit of longer objects is usually meters.

4. When measuring: aim the "0" scale of the ruler at the left end of the object, and then see how many centimeters the right end of the paper money faces.

5. 1 m = 100 cm,100 cm =1m. When calculating the length unit, first look at whether the unit is the same. If it is different, first change the unit into the same unit and then add and subtract. For example:1m-40cm = 60cm (100cm-40cm = 60cm).

6. Features of the line segment: The line segment is straight. A line segment has two endpoints. You can measure the length of a line segment.

Second, knowledge point two

1. Draw a line segment from the scale (0) of the ruler to the number required by the topic. For example, it is required to draw a 5 cm long line segment. 0 starts and 5 ends. For example, the scale from 0 to 7 is (7) centimeters. Just use 7-0=7 cm. Fill in 7 cm in the brackets. Two to eight equals six centimeters. Just use 8-2=6 cm. Fill in 6 cm in the brackets.

2. Draw a line segment that is 3 cm shorter than 6 cm. How much shorter is 3 cm than 6 cm? 6-3=3 cm. So the topic requirement is to draw a 3 cm long line segment. Example: draw a figure surrounded by three lines at will. Is to draw a triangle.

Third, knowledge point three

Even increase or decrease

1. Calculation method of continuous addition: When calculating continuous addition, it should be done from left to right, first calculate the sum of the first two numbers, and then add it to the third number.

2. Calculation method of continuous subtraction: When calculating continuous subtraction, proceed from left to right. First calculate the difference between the first two numbers, and then subtract the third number from the obtained number.

Addition-subtraction mixing

Calculation method of addition and subtraction: from left to right, first add (or subtract) the first two numbers, and then subtract (or add) the third number.