Circles

Radius & Diameter

100

A circle has a radius of 6 cm.
Find the diameter.

d=2r=2(6)=12 cm

200

Question: A circle has a diameter of 18 cm. Find the radius.

9 cm

300

A circle has a radius of 11 cm. Find the diameter.

d=2(11)=22 cm

400

A circle has a diameter of 30 m. Find the radius.

15 m

500

A circle has a radius of 15 cm. Find the diameter.

d=2(15)=30 cm

Circumference

100

Diameter = 10 cm. Find circumference.

C=3.14(10)=31.4 cm

200

Radius = 8 cm. Find circumference.

C=50.24 cm

300

Diameter = 22 cm. Find circumference.

C=3.14(22)=69.08 cm

400

Radius = 12 cm. Find circumference.

C=75.36 cm

500

Diameter = 50 m. Find circumference.

C=157 m

Area

100

Radius = 4 cm. Find area.

A=50.24 cm2

200

Radius = 6 cm. Find area.

A=113.04 cm2

300

Diameter = 12 cm. Find area.

A=113.04 cm2

400

Radius = 10 cm. Find area.

A=314 cm2

500

Radius = 14 m. Find area.

A=615.44 m2

Reverse Problems

100

Diameter = 14 cm. Find radius.

r=14/2=7 cm

200

Circumference = 31.4 cm. Find diameter.

d=10 cm

300

Area = 78.5 cm². Find radius.

r=5 cm

400

Circumference = 62.8 cm. Find radius.

r=10 cm

500

Area = 314 cm². Find radius.

r=10 cm

Thinking / Concepts

100

If the radius doubles, what happens to the diameter?

Diameter doubles.

200

If the diameter triples, how does the circumference change?

Circumference becomes 3 times larger.

300

If the radius doubles, how does the area change?

A=pi(4r2)

400

If the radius triples, how does the area change?

A=pi(9r2)

500

The diameter doubles. What happens to circumference and area?

Circumference 2× larger
Area 4× larger.