Origin

• The origin is the centerof the circle.

• All points on a circle are the same distance from the origin.

• A circle is named by its center.

• Name: Circle A

origin

A

Diameter• The diameter is the

distance of a line segment going across a circle throughits center. AB

• It divides the circle exactly in half.

• Is viewed as a line of symmetry.

• Symbol is lower case d.

diameter

Radius• The radius is the distance

from the center of the circle to any point on the circle.

• The radius is one-half the length of the diameter.

• Symbol is lower case r.

Circumference

• The circumferencerefers to the total distance around the outside edge of a circle.

• It is like the perimeter of a circle.

• Symbol is an upper case C.

Ratio of the Circumference of A Circle to Its Diameter

• If you measure the distance around a circle (C)and divide it by the distance across the circle through its center (d), you should always come close to a particular value

• We use the Greek letter to represent this value.

(pi)

Ratio Of The Circumference Of A Circle To Its Diameter

(pi)

How Helps

• Knowing the value of , allows us to use a formula to calculate the circumference.

• If the diameter of a circle is 2 cm, how could you calculate the circumference?

• C = x ___

• Estimate the circumference

• The circumference is ____

2cm

Circumference of a Circle

If the diameter is

3cm

Estimate the area of this circle.

Seeing the square units can help.

Remember each

“block” is one square

unit

Estimatedarea?

EstimatedArea?

Counting square units gives a

good estimate.

The formula for finding the area of a circle is

𝐴 = 𝜋𝑟2

Counting is not exact.

ActualArea?

Pie are square?

NO, pie are round!

Estimatedarea is?

Actual area is?

Remember

𝐴 = 𝜋𝑟2

Actual area?

Estimated area?