Perimeter (II)
The ratio of the circumference to the diameter is a fixed number called pi (π), taken as 3.14 or 22/7; circumference = π × diameter = 2 × π × radius; finding the diameter and radius from the circumference; the perimeters of semicircles and quarter circles, remembering the straight sides; the perimeters of compound shapes such as running tracks; and word problems.
Key words
| Chinese | English |
|---|---|
| 圓周率 | pi (π) |
| 圓周 | circumference |
| 直徑 | diameter |
| 半徑 | radius |
| 半圓 | semicircle |
| 周界 | perimeter |
The 4 short lessons and the chapter test
Lessons are in Chinese for now.
- 認識圓周率
- 已知半徑或圓周
- 半圓和四分一圓的周界
- 組合圖形和應用題
- 章節測驗 (chapter test)
Sample questions (in Chinese)
Question 1圓周率 π 最接近哪一個數？
- 0.314
- 3.14
- 31.4
- 2
Answer and explanation
答案：B．3.14
為甚麼？圓周大約是直徑的 3.14 倍，所以 π ≈ 3.14，也可以用 227。
Question 2求這個圓的圓周。（取 π = 3.14）
m
Answer and explanation
答案：109.9 m
為甚麼？圓周 = π × 直徑 = 3.14 × 35 = 109.9 m。
Question 3求這個圓的圓周。（取 π = 227）
m
Answer and explanation
答案：44 m
為甚麼？直徑 = 2 × 7 = 14 m。圓周 = π × 直徑 = 2 × π × 半徑 = 227 × 14 = 44 m。
Question 4一個圓的圓周是 62.8 cm。它的半徑是多少？（取 π = 3.14）
cm
Answer and explanation
答案：10 cm
為甚麼？圓周 = π × 直徑，所以直徑 = 圓周 ÷ π = 62.8 ÷ 3.14 = 20 cm。半徑 = 20 ÷ 2 = 10 cm。
Based on learning unit 6M3 of the Education Bureau’s Primary Mathematics Curriculum (2017).