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Arcs & Sectors (DP IB Maths: AI HL)
Revision Note
Length of an Arc
What is an arc?
- An arc is a part of the circumference of a circle
- It is easiest to think of it as the crust of a single slice of pizza
- The length of an arc depends of the size of the angle at the centre of the circle
- If the angle at the centre is less than 180° then the arc is known as a minor arc
- This could be considered as the crust of a single slice of pizza
- If the angle at the centre is more than 180° then the arc is known as a major arc
- This could be considered as the crust of the remaining pizza after a slice has been taken away
How do I find the length of an arc?
- The length of an arc is simply a fraction of the circumference of a circle
- The fraction can be found by dividing the angle at the centre by 360°
- The formula for the length, , of an arc is
-
- Where is the angle measured in degrees
- is the radius
- This is in the formula booklet, you do not need to remember it
Examiner Tip
- Make sure that you read the question carefully to determine if you need to calculate the arc length of a sector, the perimeter or something else that incorporates the arc length!
Worked example
A circular pizza has had a slice cut from it, the angle of the slice that was cut was 38 °. The radius of the pizza is 12 cm. Find
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Area of a Sector
What is a sector?
- A sector is a part of a circle enclosed by two radii (radiuses) and an arc
- It is easier to think of this as the shape of a single slice of pizza
- The area of a sector depends of the size of the angle at the centre of the sector
- If the angle at the centre is less than 180° then the sector is known as a minor sector
- This could be considered as the shape of a single slice of pizza
- If the angle at the centre is more than 180° then the sector is known as a major sector
- This could be considered as the shape of the remaining pizza after a slice has been taken away
How do I find the area of a sector?
- The area of a sector is simply a fraction of the area of the whole circle
- The fraction can be found by dividing the angle at the centre by 360°
- The formula for the area, , of a sector is
-
- Where is the angle measured in degrees
- is the radius
- This is in the formula booklet, you do not need to remember it
Worked example
Jamie has divided a circle of radius 50 cm into two sectors; a minor sector of angle 100° and a major sector of angle 260°. He is going to paint the minor sector blue and the major sector yellow. Find
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Arcs & Sectors Using Radians
How do I use radians to find the length of an arc?
- As the radian measure for a full turn is , the fraction of the circle becomes
- Working in radians, the formula for the length of an arc will become
- Simplifying, the formula for the length, , of an arc is
-
- is the angle measured in radians
- is the radius
- This is given in the formula booklet, you do not need to remember it
How do I use radians to find the area of a sector?
- As the radian measure for a full turn is , the fraction of the circle becomes
- Working in radians, the formula for the area of a sector will become
- Simplifying, the formula for the area, , of a sector is
-
- is the angle measured in radians
- is the radius
- This is given in the formula booklet, you do not need to remember it
Worked example
A slice of cake forms a sector of a circle with an angle of radians and radius of 7 cm. Find the area of the surface of the slice of cake and its perimeter.
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