Name: ________________
Date: _________
Aim: How do we find the area & circumference of a circle?
Do Now: Define the following and label on the circle.
Diameter___________________________________
Radius_____________________________________
Circumference_______________________________
1. The diameter of a circle is 8 inches.
Find the radius.
2. The radius of a circle is 15 cm.
Find the diameter.
Geo
Complete the following chart using the information from the class circleactivity:
Diameter Circumference
Diameter _______ = Circumference
50
40
30
20
10
Circumference formulas :
π = _________
Practice:
Find the circumference of a circle when :
1. the radius is 4, in terms of π .
2. the diameter is 14,
to the nearest tenth.
3. Find the radius of a circle, to the
nearest tenth, when the
circumference is 40 .
4. Find the diameter of a circle when
the circumference is 20 π .
Area of a Circle
GeoLink
Area
Area of
Rectangle
Substitute
r and C
Plug in
Circumference
Formula
Solve
Area of a Circle Formula
Practice:
Find the area of a circle when...
4. the radius is 4, to the nearest tenth.
6. the diameter is 12, to the
nearest hundredth.
8. If the circumference is 12π,
find the radius.
5. the diameter is 14, in terms of π.
7. the radius is 12, in terms of π.
9. If the area is 64π, find the diameter.
11. If the circumference is 10π, find the area, in terms of π.
12. If the area is 9π, find the circumference, in terms of π. 13. The side of a square is 20 cm long. What is the circumference of the circle when …
a. the circle is inscribed within the square?
b. the square is inscribed within the circle?
14. A designer needs to create prefectly circular necklaces. The necklaces each need
to have a radius of 8 inches. What is the largest number of necklaces that can be
made from 2000 inches of wire?
15. The stained glass window below is a semi circle.
To the nearest tenth, what is the distance around the window?
16. The cost of painting the circular traffic sign shown below is $2.75 per
square foot. How much, to the nearest cent, will it cost to paint the sign if its
diameter is 30 inches?
Sum it Up
• The area of a circle is πr .
• The circumference of a circle is πd or 2πr.
2