Circles
The centre, radius, diameter and circumference of a circle; every point on a circle is the same distance from the centre; a diameter is twice a radius, and a circle has countless radii and diameters, all the same length; the diameter is the longest segment joining two points on a circle; drawing circles with compasses, where the gap between the points is the radius; and problems with circles in a row.
Key words
| Chinese | English |
|---|---|
| 圓心 | centre |
| 半徑 | radius |
| 直徑 | diameter |
| 圓周 | circumference |
| 圓規 | compasses |
The 4 short lessons and the chapter test
Lessons are in Chinese for now.
- 圓心、半徑和直徑
- 無數條半徑和最長的線段
- 用圓規畫圓
- 圓的性質
- 章節測驗 (chapter test)
Sample questions (in Chinese)
Question 1圖中的點 O 是圓的甚麼？
- 直徑
- 半徑
- 圓心
- 圓周
Answer and explanation
答案：C．圓心
為甚麼？圓的中心點 O 叫做圓心。
Question 2P 和 G 都是圓上的點，O 是圓心。OP = 11 厘米，OG 長多少厘米？
OG = 厘米
Answer and explanation
答案：OG = 11 厘米
為甚麼？圓上所有的點跟圓心的距離相同，所以 OG = OP = 11 厘米。
Question 3一個圓的直徑是 29 厘米。它的半徑是多少厘米？
半徑 = 厘米
Answer and explanation
答案：半徑 = 14.5 厘米
為甚麼？半徑是直徑的一半：29 ÷ 2 = 14.5 厘米。
Question 4圖中的三條線段，兩端都在圓上。哪一條最長？
- EA
- DP
- FC
- 一樣長
Answer and explanation
答案：C．FC
為甚麼？以圓上任何兩點為端點的線段中，經過圓心的線段（直徑）最長。FC 經過圓心 O，所以最長。
Based on learning unit 5S1 of the Education Bureau’s Primary Mathematics Curriculum (2017).