(2) From the rotation of line segment to the rotation of figure, let students learn to describe the rotation process with the center point, direction and rotation angle. Specific practices:
Please observe the rotation process of the pointer carefully. (The pointer points from 12 to 1) Who can describe the rotation process completely in one sentence? 180 if the pointer continues to rotate clockwise around the O point from "6". What will it point to?
We have described so many rotation phenomena. Think about it. What should we say to describe a rotation phenomenon clearly?
③ Group activities to explore the rotating characteristics of windmills.
How did the windmill change from figure 1 to figure 2? Let the students work in groups.
* * * Solve the problem together.
From figure 1 to figure 2, the windmill rotates counterclockwise _ _ _ degrees around point O.
How to judge the angle of windmill rotation? Through observation, we found that after the windmill rotates, not only each triangle rotates 90 counterclockwise around the O point, but also each line segment and each vertex rotates 90 counterclockwise around the O point. )
④ Reveal the characteristics and properties of rotation.
From the picture, we can clearly see that the position of each triangle has changed after the windmill rotates, so what hasn't changed? The shape and size of the triangle have not changed. Health 2: The position of point O has not changed. The length of the corresponding line segment has not changed. The included angle of the corresponding line segment has not changed. )
If we rotate the windmill on the basis of Figure 2, continue to rotate counterclockwise around point O 180. So where should the yellow triangle turn?
(3) In order to improve students' spatial imagination and correctly describe the process of graphic rotation, we can add a phenomenon of both clockwise and counterclockwise in the process of rotation, so that students can reasonably describe it with three elements on the basis of correct analysis. So as to consolidate students' understanding of rotation transformation.
2. The understanding of the meaning of rotation and the application of the characteristics and properties of rotation make this unit more difficult. To break through this difficulty, it is best to follow the principle of from easy to difficult and from special to general in teaching, so that students can use the law of graphic rotation and master the skills of drawing rotating graphics. Specific practices:
(1) shows a right triangle with two right-angled sides coincident with the grid diagram. Ask the students to think independently about how to rotate the triangle 90 degrees clockwise and draw the rotated pattern. On the basis of independently completing the requirements, through communication and sharing strategies: rotate 90 clockwise, OA and OA' are perpendicular to each other, OA = OA', OB and OB' are perpendicular to each other, OB = OB' (point B and point B' can also be symmetrical), and connect A' and B'.
(2) Example 4, completed independently according to the strategy just now. What do you find by comparing these two questions? (It's a little difficult to find the line perpendicular to OB)
(3) Show a triangle whose three sides are not coincident with the grid diagram, think about how to rotate the triangle 90 degrees clockwise and draw the rotated pattern. When communicating at the same table, there may be a strategy of using a protractor or a right triangle to help. ) After the teacher affirms these practices, students can think: Why is it easy to rotate when the two sides overlap with the grid diagram, and you can draw while you can, but if you don't overlap, you have to use tools? After thinking, we can tell students a relatively simple method: draw a rectangle whose four sides coincide with the grid diagram on the periphery of the figure to be rotated, which can be regarded as rectangular rotation, then find out the symmetrical points from the rectangle and finally connect the lines. In this way, the method of drawing a graph with arbitrary rotation of 90 degrees is obtained.
3, warm tips
(1) P6Do it1(Ask the students to clarify "which figure rotates around which point" and "in which direction". ) p8 Exercise 1, Question 3, Band 1. (2)p6 does one thing and does two things, p8 questions 4 and 6 (Let students discover the characteristics of another kind of graph "rotational symmetry graph" through experiments. These figures rotate around their center at a certain angle and coincide with the original figures. It is not necessary for students to understand the concept of "rotationally symmetric figures", as long as students can describe the characteristics of these figures overlapping with the original figures after 360 degrees rotation in their own language. The key is to guide how to find the center point. ) Cooperate with the second class. (3) Draw a parallelogram that rotates 90 degrees counterclockwise to improve students' ability to draw rotating figures.