LESSON

Date

Class

Reteach

Developing Formulas for Circles and Regular Polygons

Circumference and Area of Circles

9-2

A circle with diameter d and radius r has circumference C d or C 2 r. A circle with radius r has area A

2 r .

Find the circumference of circle S in which A Step 1 Use the given area to solve for r. A 81 cm

2

2 81 cm .

r2 r r2 r

2

Area of a circle Substitute 81 for A. Divide both sides by . Take the square root of both sides. cm 81 cm2 9 cm Step 2

Use the value of r to find the circumference. C C 2 r 2 (9 cm) 18 cm Circumference of a circle Substitute 9 cm for r and simplify.

Find each measurement. 1. the circumference of circle B 2. the area of circle R in terms of

6 – cm

5m

C

6 cm

A

25 m2

3. the area of circle Z in terms of

4. the circumference of circle T in terms of

22 ft

10 in.

A

121 ft 2

C

20 in.

18 cm

5. the circumference of circle X in 2 which A 49 in

6. the radius of circle Y in which C

C

Copyright © by Holt, Rinehart and Winston. All rights reserved.

14 in.

14

r

9 cm

Holt Geometry

Name

LESSON

Date

Class

Reteach

Developing Formulas for Circles and Regular Polygons continued

Area of Regular Polygons The center is equidistant from the vertices.

9-2

The area of a regular polygon with apothem a and perimeter P 1 is A __aP. 2

The apothem is the distance from the center to a side.

Find the area of a regular hexagon with side length 10 cm. Step 1 Draw a figure and find the measure of a central angle. Each central 360° angle measure of a regular n-gon is ____. n A central angle has its vertex at the center. This central angle measure is 360 ____ 60 . n Step 2 Use the tangent ratio to find the apothem. You could also use the 30°-60°-90° Thm. in this case. tan 30° tan 30° leg opposite 30° angle ____________________ leg adjacent to 30° angle 5 cm _____ Write a tangent ratio. Substitute the known values. Solve for a.