**Circle**

**Circle** Circle

A circle is easy to make:

And so:

All points are the same distance from the center.

A circle is easy to make:

*Draw a curve that is "radius" away*

from a central point.from a central point.

And so:

All points are the same distance from the center.

You Can Draw It YourselfPut a pin in a board, put a loop of string around it, and insert a pencil into the loop. Keep the string stretched and draw the circle!

## Radius, Diameter and Circumference.

The

The

And here is the really cool thing:

When you divide the circumference by the diameter you get 3.141592654...

which is the number π (Pi)

So when the diameter is 1, the circumference is 3.141592654...

**Radius**is the distance from the center to the edge.The**Diameter**starts at one side of the circle, goes through the center and ends on the other side.The

**Circumference**is the distance around the edge of the circle.And here is the really cool thing:

When you divide the circumference by the diameter you get 3.141592654...

which is the number π (Pi)

So when the diameter is 1, the circumference is 3.141592654...

We can say:

Circumference =

Example: You walk around a circle which has a diameter of 100m, how far have you walked?Distance walked = Circumference = π × 100m

=

Circumference =

**π**× DiameterExample: You walk around a circle which has a diameter of 100m, how far have you walked?Distance walked = Circumference = π × 100m

=

**314m**(to the nearest m)Also note that the Diameter is twice the Radius:

Diameter = 2 × Radius

And so this is also true:

Circumference = 2 ×

RememberingThe length of the words may help you remember:

Diameter = 2 × Radius

And so this is also true:

Circumference = 2 ×

**π**× RadiusRememberingThe length of the words may help you remember:

**Radius**is the shortest word**Diameter**is longer (and is 2 × Radius)**Circumference**is the longest (and is π × Diameter)

NamesBecause people have studied circles for thousands of years special names have come about.

Nobody wants to say

So here are the most common special names:

Nobody wants to say

*"that line that starts at one side of the circle, goes through the center and ends on the other side"*when a word like*"Diameter"*would do.So here are the most common special names:

LinesA line that goes from one point to another on the circle's circumference is called a

If that line passes through the center it is called a

A line that "just touches" the circle as it passes by is called a

And a part of the circumference is called an

**Chord**.If that line passes through the center it is called a

**Diameter**.A line that "just touches" the circle as it passes by is called a

**Tangent**.And a part of the circumference is called an

**Arc**. Slices

There are two main "slices" of a circle

The "pizza" slice is called a

And the slice made by a chord is called a

There are two main "slices" of a circle

The "pizza" slice is called a

**Sector**.And the slice made by a chord is called a

**Segment**.