1. 10.5 Areas of Circles and Sectors
Geometry

2. Objectives/Assignment:
Find the area of a circle and a sector of a circle.
Use areas of circles and sectors to solve real-life problems such as finding the areas of portions of circles.

3. Areas of Circles and Sectors
The diagrams on the next slide show regular polygons inscribed in circles with radius r. The number of sides increases, the area of the polygon approaches the value r2.

4. Examples of regular polygons inscribed in circles.

5. Theorem. 11.7: Area of a Circle
The area of a circle is  times the square of the radius or A = r2.

6. Ex. 1: Using the Area of a Circle
Find the area of P.
Solution:
Use r = 8 in the area formula.
A = r2
=  • 82
= 64 
So, the area if 64, or about square inches.

7. Ex. 1: Using the Area of a Circle
Find the diameter of Z.
Solution:
Area of circle Z is 96 cm2.
A = r2
96=  r2
96= r2 
30.56  r2
5.53  r
The diameter of the circle is about cm.

8. More . . .
A sector of a circle is a region bounded by two radii of the circle and their intercepted arc. In the diagram, sector APB is bounded by AP, BP, and The following theorem gives a method for finding the area of a sector.

9. Theorem 11.8: Area of a Sector
The ratio of the area A of a sector of a circle to the area of the circle is equal to the ratio of the measure of the intercepted arc to 360°. m m A = or A =
• r2
r2
360°
360°

10. Ex. 2: Finding the area of a sector
Find the area of the sector shown below.
Sector CPD intercepts an arc whose measure is 80°. The radius is 4 ft. m A =
• r2
360°

11. Ex. 2 Solution Write the formula for area of a sector.
360° =
• r2
Write the formula for area of a sector.
80°
360° =
• r2
Substitute known values.
Use a calculator.
 11.17
So, the area of the sector is about square feet.

12. Ex. 3: Finding the Area of a Sector
A and B are two points on a P with radius 9 inches and mAPB = 60°. Find the areas of the sectors formed by APB.
FIRST draw a diagram of P and APB. Shade the sectors.
LABEL point Q on the major arc.
FIND the measures of the minor and major arcs.
60°

13. Ex. 3: Finding the Area of a Sector
Because mAPB = 60°, m = 60° and
m = 360° - 60° = 300°.
Use the formula for the area of a sector.
60°
360° =
• r2
Area of small sector
60° =
•  • 92
360° 1 =
•  • 81 6
 square inches

14. Ex. 3: Finding the Area of a Sector
Because mAPB = 60°, m = 60° and
m = 360° - 60° = 300°.
Use the formula for the area of a sector.
300°
Area of large sector =
• r2
360°
60° =
•  • 92
360° 5 =
•  • 81 6
 square inches

15. Using Areas of Circles and regions
You may need to divide a figure into different regions to find its area. The regions may be polygons, circles, or sectors. To find the area of the entire figure, add or subtract the areas of separate regions as appropriate.

16. Ex. 4: Find the Area of a Region
Find the area of the region shown.
The diagram shows a regular hexagon inscribed in a circle with a radius of 5 meters. The shaded region is the part of the circle that is outside the hexagon.
Area of
Circle
Area of
Hexagon
Area of
Shaded =

17. Solution: = = = ( • 52 ) – ½ • ( √3) • (6 • 5)
Area of
Circle
Area of
Hexagon
Area of
Shaded = =
r2
½ aP
= ( • 52 ) – ½ • ( √3) • (6 • 5) 75
= 25 √3, or about square meters. 2

18. Ex. 5: Finding the Area of a Region
Woodworking. You are cutting the front face of a clock out of wood, as shown in the diagram. What is the area of the front of the case?

19. Ex. 5: Finding the Area of a Region

20. More . . .
Complicated shapes may involve a number of regions. In example 6, the curved region is a portion of a ring whose edges are formed by concentric circles. Notice that the area of a portion of the ring is the difference of the areas of the two sectors.