


Circles
Circles are another popular plane geometry test topic.
Unlike polygons, all circles are the same shape and vary only in
size. Circles have certain basic characteristics, and test questions
will focus on your understanding of these properties.
Basic Definitions of a Circle
A circle is the collection of all points equidistant from
a given point, called the center. A circle is named after its center
point. The distance from the center to any point on the circle is called
the radius, r, and is the most important measurement
in a circle. If you know the radius of a circle, you can figure
out all of its other characteristics. The diameter d of
a circle is twice as long as the radius (d = 2r)
and stretches between endpoints on the circle, making sure to pass
through the center. A chord also extends from endpoint to endpoint
on the circle but does not pass through the center. In the figure
below, point C is the center of the circle, r is
the radius, and AB is a chord.
Tangent Lines
A line that intersects the circle at exactly one point
is called a tangent line. The radius whose endpoint is the intersection
point of the tangent line and the circle is always perpendicular
to the tangent line.
Every point in space outside the circle can extend exactly
two tangent lines to the circle. The distance from the origin of
the two tangents to the points of tangency are always equal. In
the following figure, XY = XZ:
Central Angles and Inscribed Angles
An angle whose vertex is the center of the circle is called
a central angle.
The degree of the circle (the slice of pie) cut by a central
angle is equal to the measure of the angle. If a central angle is
25º, then it cuts a 25º arc in the circle.
An inscribed angle is an angle formed by two chords in
a circle that originate from a single point.
An inscribed angle will always cut out an arc in the circle
that is twice the size of the degree of the inscribed angle. If
an inscribed angle has a degree of 40º, it will cut an arc of 80º
in the circle.
If an inscribed angle and a central angle cut out the
same arc in a circle, the central angle will be twice as large as
the inscribed angle.
Circumference of a Circle
The circumference of a circle is the length of the 360º
arc that forms the circle. In other words, if you were to trace
around the edge of the circle, it is the distance from a point on the
circle back to itself. The circumference is the perimeter of the
circle. The formula for circumference is
where r is the radius. The formula can
also be written C = d, where d is
the diameter. Using the formula, try to find the circumference of
the circle below:
Plugging the radius into the formula, C =
2r = 2 (3) = 6.
Arc Length
An arc is part of a circle’s circumference. An arc contains
two endpoints and all of the points on the circle between the endpoints.
By picking any two points on a circle, two arcs are created: a major
arc, which is by definition the longer arc, and a minor arc, the
shorter one.
Since the degree of an arc is defined by the central or
inscribed angle that intercepts the arc’s endpoints, you need only
know the measure of either of those angles and the measure of the
radius of the circle to calculate the arc length. The arc length
formula is:
where n is the measure of the degree
of the arc and r is the radius. The formula could
be rewritten as arc length = ^{n}
/_{360} C, where C is
the circumference of the circle.
Area of a Circle
The area of a circle depends on the radius of the circle.
The formula for area is written:
where r is the radius. If you know the
radius, you can always find the area.
Area of a Sector
A sector of a circle is the area enclosed by a central
angle and the circle itself. It’s shaped like a slice of pizza.
The shaded region in the figure below is a sector:
The area of a sector is related to the area of a circle
the same way that the length of an arc is related to circumference.
To find the area of a sector, simply find what fraction of 360º
the sector constitutes and multiply this fraction by the area of
the circle.
where n is the measure of the central
angle that forms the boundary of the sector and r is the
radius.
Polygons and Circles
The Math IIC test occasionally contains questions that
combine polygons and circles in the same figure. Here’s an example:

To find the length of major arc BE, you
must know two things: the measure of the central angle that intersects
the circle at its endpoints and the radius of the circle. Because ABCD is a
rectangle and rectangles only have right angles, figuring out the
measure of the central angle is simple. Angle BAD is
90º, so the measure of the central angle is 360º – 90º = 270º.
Finding the radius of the circle is a little tougher.
From the diagram, you can see that it is equal to the height of
the rectangle. To find the height of the rectangle, you can use
the fact that the area of the rectangle is 18 and the length is
6. Since A = bh and you know both a and b, h = Ab = 186 = 3. With a radius of 3, we can
use the arc length formula to find the length of arc BE. BE
= ^{270}/_{360} 2(3) = ^{3}
/_{4} 6 = ^{9π}
/_{2} .
