3. What is circle? Parts of it.
A circle is a set of points equidistant from its center. Equidistant means “the
same distance”.
The circumference (C) is the distance around the circle.
The radius (r; R) is the distance from the center to the circumference.
The diameter (d; D) is the distance across the center of the circle.
Part of the circumference is called an arc.
4. A chord is a line joining two points on the circumference.
A segment is the region enclosed between a chord and an arc.
A sector is the region enclosed by an arc and two radii.
5. Circumference of the circle
The perimeter of the circle is called the circumference.
The diameter is twice length of the radius: 𝑑 = 2 × 𝑟
𝑪𝒊𝒓𝒄𝒖𝒎𝒇𝒆𝒓𝒆𝒏𝒄𝒆 = 𝝅 × 𝒅𝒊𝒂𝒎𝒆𝒕𝒆𝒓 𝑜𝑟 𝑪 = 𝝅𝒅
𝑪𝒊𝒓𝒄𝒖𝒎𝒇𝒆𝒓𝒆𝒏𝒄𝒆 = 𝝅 × 𝟐 × 𝒓𝒂𝒅𝒊𝒖𝒔 𝑜𝑟 𝑪 = 𝟐𝝅𝒓, 𝝅 = 𝟑. 𝟏𝟒𝟏𝟓𝟗𝟐 …
6. Examples
• The circle has diameter 14 inches. What is its circumference? Use
22
7
as
an approximation for 𝜋.
• The circle has radius 21 centimeters. What is its circumference? Use
22
7
as an approximation for 𝜋.
• A circular pond has a radius of 10 feet. What is the circumference of the
pond? Use 3.14 as an approximation for 𝜋.
• A circular garden has a radius of 21 m. the owner wants to put a plastic
edge around the garden, so wants to know what is the circumference of
the garden? Use
22
7
as an approximation for 𝜋.
7. Area of a circle
𝐴𝑟𝑒𝑎 𝑜𝑓 𝑎 𝑐𝑖𝑟𝑐𝑙𝑒 = 𝜋 × 𝑟𝑎𝑑𝑖𝑢𝑠 × 𝑟𝑎𝑑𝑖𝑢𝑠
𝐴𝑟𝑒𝑎 = 𝜋𝑟2
𝐴𝑟𝑒𝑎 = 𝜋
𝑑
2
2
= 𝜋
𝑑2
4
Example: Calculate the area of a circle. Radius is 9 cm and take 𝜋 to be 3.14.
Solution: 𝐴𝑟𝑒𝑎 = 𝜋𝑟2 = 3.14 × 9 × 9 = 254.34 𝑐𝑚2
8. Examples
• A circular table has diameter 1.4 m. What is the area of the top of the
table? Use
22
7
as an approximation for 𝜋.
• The circle has radius 21 centimeters. What is its area? Use
22
7
as an
approximation for 𝜋.
• The circle has diameter 14 inches. What is its area? Use
22
7
as an
approximation for 𝜋.
• The radius of the circle is 16 mm. What is its area? Take 𝜋 to be 3.14.
