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Area of Circles, Sectors and Segments

A series of free High School Geometry Video Lessons from Brightstorm.

Area of Circles
The area of circles is derived by dividing a circle into an infinite number of wedges formed by radii drawn from the center. When these wedges are rearranged, they form a rectangle whose height is the radius of the circle and whose base length is half of the circumference of the circle. The area of circles are also used in sectors, segments and annuluses.

Area of a Sector
A sector in a circle is the region bound by two radii and the circle. Since it is a fractional part of the circle, the area of any sector is found by multiplying the area of the circle, pi*r^2, by the fraction x/360, where x is the measure of the central angle formed by the two radii. The area of a sector is also used in finding the area of a segment.

Area of a Segment
The area of a segment in a circle is found by first calculating the area of the sector formed by the two radii and then subtracting the area of the triangle formed by the two radii and chord (or secant). In segment problems, the most challenging aspect is often calculating the area of the triangle. Related topics include area of a sector, area of a circle and area of an annulus.

Area of an Annulus
An annulus is similar to a ring or a castle's moat; it is the area between two concentric circles. Calculating the annulus area, therefore, involves finding the difference of the two circles' area. A common trick on annulus problems is to give the distance between the small and large circle, and not the large circle's radius. Related topics include area of a sector, area of a circle and area of an segment.

Regions Between Circles and Squares
A common application of the area of a circle and the area of a square are problems where a circle is circumscribed about a square or inscribed in a square. Regions between circles and squares problems almost always involve subtracting the two areas; their difficulty stems from dimensions given for one but not both shapes. Related topics include area of sectors and area of circles.

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