Ask Dr. Math

Circles

Browse Elementary Circles
Stars indicate particularly interesting answers or good places to begin browsing.

Selected answers to common questions:
    Circle formulas - area, circumference.
    Is a circle a polygon?
    Why is a circle 360 degrees?

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Circle Area and Circumference [03/16/2002]

Are radius and diameter the same as area and circumference?

Circle Formulas: Area and Perimeter [03/19/2003]

I get confused with the different formulas, such as Pi r squared or Pi d (Pi times radius squared or Pi times diameter).

Circumference [04/12/1997]

What is the circumference of a circle with a radius of 5.5?

Circle Definitions [04/29/2002]

Define a circle and its parts: diameter, radius, center, circumference.

Circumference and Diameter [8/10/1996]

If you know the circumference of a circle, how do you find the diameter?

Clockwise and Counterclockwise [03/19/2001]

What does clockwise mean?

Deriving the Area Formula for a Circle [03/08/2000]

Why is the area of a circle the square of the radius times pi?

Dividing a Circle [5/29/1996]

If pi is 3.141..., can a circle truly be separated into 4 equal parts?

Eratosthenes and the Circumference of the Earth [10/7/1995]

How did Eratosthenes measure the circumference of the earth?

Explorations of the Circle [02/09/1999]

What is the definition of a circle? How does it relate to pi? What are the important parts of a circle?

Intersection of Circles [10/08/1998]

I do not understand the term intersection in geometry.

Perimeter of an Oval [04/08/1999]

What is the perimeter of an oval called?

Simple Closed Curve [05/22/2001]

Why is a simple closed curve called just that?

Three Cuts and Seven Pieces [01/18/1999]

Is there any way that you can cut a pie into seven pieces with just three straight cuts?

What is Circumference? [06/25/2001]

My friend told me it's somewhat like the measurement of the outside of a circle, but I don't understand that.

What is the difference between these two Pi formulas? [01/30/2001]

To find the circumference of a circle, I know how to use Pi*Diameter and 2*Pi*R, but people have told me about Pi*R*squared. What is that?

Why Does 1 Degree Equal 60 Minutes? [07/28/2004]

In 60 minutes, the minute hand of a clock makes a full circle, which is 360 degrees. How can 60 minutes equal 1 degree when measuring angles?

Why Is a Circle 360 Degrees? [07/01/1998]

Why is a circle defined as 360 degrees?

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