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随着新课程的实施和素质教育的不断深入,一些与几何有关的动态变化题已成为近年来中考数学的热点之一．其中又以圆的动态变化最为丰富多彩．本文谈谈中考题中的“圆的移动问题．”圆的移动是指圆心按某个条件运动,而圆的大小在运动过程中保持不变．一、在直线上的移动例1 (2005年南京市中考)如图1,形如量角器的半圆O的直径DE=12 cm,形如三角板的△ABC中,∠ACB=90°,∠ABC=30°,BC =12 cm．半圆O以2 cm／s的速度从左向右运动,在运动过程中,点D、E始终在直线BC上．
With the implementation of the new curriculum and the continuous deepening of quality education, some dynamic problems related to geometry have become one of the hot topics in mathematics in recent years. Among them, the dynamic change of the circle is the most colorful. This article talks about the “circular movement problem” in the exam. The circle movement means that the center of the circle moves according to certain conditions, and the size of the circle remains unchanged during the movement. 1. Movement on the straight line Example 1 (Nanjing City Entrance Examination in 2005) As shown in Fig. 1, the diameter of a semicircle O shaped like a protractor is DE=12 cm, in the shape of a triangle ABC, ∠ACB=90°, ∠ABC= 30°, BC =12 cm. The semicircle O moves from left to right at a speed of 2 cm/s. During the movement, points D and E are always on the straight line BC.