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Circumference and area of circles
Pick up your calculator for today’s notes
Radius
CenterDiameter
Circumference
dd = 2= 2r r oror r = r = d d ÷ 2÷ 2
The parts of a circle
Pi () is an irrational number that is often approximated by the rational numbers 3.14 and .22
7
The circumference of a circle is the distance around the circle.
The area of a circle is the number of square units needed to cover the circle.
A. Circle with a radius of 4 m C = 2r= 2(4)= 8m
B. Circle with a diameter of 3.4 ft C = d= (3.4)
= 3.4ft
Find the circumference of each circle, both in terms of and to the nearest hundredth. Use 3.14 for .
= 25.12 m= 8 * 3.14
= 10.676 ft
= 3.4 * 3.14
= 10.68 ft
4m
3.4 ft
A = (42)
= 50.24 in2
A. Circle with a radius of 4 in.
Find the area of each circle, both in terms of and to the nearest hundredth. Use 3.14 for .
B. Circle with a diameter of 3.4 m
(1.72)
= 2.89 m2
d2 = 1.7
A = r2
= 16in2
= 16 * 3.14
A = r2
= 9.07 m2
= 9.0746 m2
= 2.89 * 3.14
4in
3.4 m
C = d
= 176 ft
= (56) 22 7
22 7
What is the circumference of a circle with a diameter of
56 feet? Use for .
56 1
22 7
= (56)
8 1
22 1
1
8
=
=
56 ft
Remember, we are multiplying so we are allowed to cancel before getting our answer