Talk more about this topic

Firstly, the relationship between the area ratio and the side length ratio of a rectangle is explored.

1. Show me two rectangles on page 52.

It is pointed out that a large rectangle is a graph obtained by enlarging a small rectangle.

Measure their length and width in the book and write the proportion of the corresponding sides.

Teacher's blackboard: length: 3: 1 width: 3: 1

2. The aspect ratio of these two rectangles is 3: 1. What is the area ratio of large rectangle to small rectangle?

Try to verify it and see if the estimate is correct.

Q: How did you verify it? What conclusion have you reached?

4. If the ratio of the corresponding sides of a large rectangle to a small rectangle is 4: 1, what is the area ratio?

Second, explore the relationship between the area and the side length ratio of other graphics.

1, showing squares, triangles and circles enlarged in proportion.

Guided observation: estimate how much their corresponding edges are enlarged?

2. What changes have taken place in the area of these figures after amplification compared with that before amplification?

(1) Guide students to guess.

(2) Guided observation: What laws have you found by observing the data in the table?

On the basis of students' full communication, the law is revealed: after the plane figure is enlarged according to the ratio of n: 1, the ratio of the enlarged area to the area before enlargement is N2: 1.

3. Extended discussion: If a graph is reduced by the ratio of 1: n, what is the change law of graph area before and after reduction?

Note: If a graph is shrunk at the ratio of 1: n, the change rule of graph area before and after shrinking is as follows:

The ratio of the area before reduction to the area after reduction is 1: N2.

Third, the application of the law.

Show the campus plan of Donggang Primary School in the book. Please select a building or facility, and measure and calculate its actual floor area.

(1) Measure the distance on the map.

(2) Calculate the actual area of related graphics.

Tell me how it was worked out.

Four. Activity summary

What did you get from the activities in this class? How did you do in the activity?

Blackboard design:

Area change

Table abbreviation

After the plane figure is enlarged according to the ratio of n: 1, the ratio of the enlarged area to the area before enlargement is N2: 1.

There are too many knowledge points in this class. Our rural children can't finish a class. I spent two classes.