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Circles have many different parts to them:
-the circumference
-the area
-an arc
-a sector
-and a segment

In all circles, the ratio of the diameter to the circumference is approximately 3.14159, symbolized by the Greek letter pi. To find the circumference of a circle, multiply the diameter by pi, or if you are just given the radius, double the radius and multiply that by pi, because the length of the diameter is twice the length of the radius.

The area of a circle is the space contained by the circumference. To find the area of a circle, multiply pi by the square of the radius. Or, if you are only given the circumference, multiply the circumference by the radius and divide the product by two. The area is always written in square units.

An arc is a curved portion of a circle. The relationship of its length to the circumference is proportional to the relationship of the angle formed by the radii of the arc to the total angle of the circle at the center. To find the arc length, divide the degree measure of the central angle by 360 and multiply the quotient by the circumference, or 2*pi*r.

A sector is the pie-shaped piece of the circle "cut out" by two radii. The ratio of the area of the sector to the area of the circle is equal to the ratio of the arc length of the sector to the circumference, which is the same as the ratio of the angle of the sector to the total angle of the circle.

A segment is either of the two regions into which a line or line segment cuts a circle.

Some Examples
Practice Problems