1. Sector Area and Arc Length
12-3
Sector Area and Arc Length
Holt McDougal Geometry
Holt Geometry

2. Learning Targets I will find the area of sectors.
I will find arc lengths.

3. Vocabulary
sector of a circle
segment of a circle
arc length

4. The area of a sector is a fraction of the circle containing the sector
The area of a sector is a fraction of the circle containing the sector. To find the area of a sector whose central angle measures m°, multiply the area of the
circle by

5. Notice the area formula for a circle in the formula for a sector: A = πr2. You simply take the entire area of the circle and multiply that result by the measure of the central angle that contains the sector or the arc measure of the sector (which are the same), then divide by 360°. It is that simple…

6. Example: Finding the Area of a Sector
Find the area of each sector. Give answers in terms of  and rounded to the nearest hundredth.
sector HGJ
= 52.4 m2  m2

7. Example 2: Automobile Application
A windshield wiper blade is 18 inches long. To the nearest square inch, what is the area covered by the blade as it rotates through an angle of 122°?
 345 in2

8. A segment of a circle is a region bounded by an arc and its chord.

9. Example 3: Finding the Area of a Segment
Find the area of segment LNM to the nearest hundredth.
Step 1 Find the area of sector LNM.
Use formula for area of sector.
Substitute 9 for r and 120 for m.
= 27 cm2
Simplify.

10. Example 3 Continued
Step 2 Find the area of ∆LNM. Draw altitude NO.

11. In a 30°-60°-90° triangle, the length of the leg opposite the 60° angle is √3 times the length of the shorter leg.
Remember!

12. Example 3 Continued
Find the area of segment LNM to the nearest hundredth.
Step 3
area of segment = area of sector LNM – area of ∆LNM
 cm2

13. In the same way that the area of a sector is a fraction of the area of the circle, the length of an arc is a fraction of the circumference (2πr) of the circle.

14. Example: Finding Arc Length
Find each arc length. Give answers in terms of  and rounded to the nearest hundredth. FG
 5.96 cm  cm

15. Example 4B: Finding Arc Length
Find each arc length. Give answers in terms of  and rounded to the nearest hundredth.
an arc with measure 62 in a circle with radius 2 m
Use formula for area of sector.
Substitute 2 for r and 62 for m.
 0.69 m  2.16 m
Simplify.

16. Check It Out! Example 4a
Find each arc length. Give your answer in terms of  and rounded to the nearest hundredth. GH
Use formula for area of sector.
Substitute 6 for r and 40 for m.
=  m  4.19 m
Simplify.

17. Check It Out! Example 4b
Find the arc length if the arc has a measure of 135° and the circle has a radius of 4 cm. Give your answer in terms of  and rounded to the nearest hundredth.
= 3 cm  9.42 cm

18. HOMEWORK:
Page 813, #2 – 10.

19. Lesson Quiz: Part II
3. The gear of a grandfather clock has a radius of 3 in. To the nearest tenth of an inch, what distance does the gear cover when it rotates through an angle of 88°?
 4.6 in.
4. Find the area of segment GHJ to the nearest hundredth.
 m2