defined by the basic equation
axbyc,in which a, b,
and care any number, but a
and b can’t be zero at the
same time.
A line segment is the
shortest curve—which is actu-
ally straight—to connect two
different points. It is a finite
portion of an infinite line. Line
segments are most often
labeled with two letters corre-
sponding to the endpoints of
the line segment, such as a
and b,and written as ab.
Distanceis the length of
the path between two points,
or the length of a segment, for
example, between points aand
b. When talking about the dis-
tance between any two points
associated with aand b(as real
numbers) on a number line,
distance becomes the absolute
value of ba(|ba|).
Another basic building
block is the ray.Think of a
ray as a laser beam: It origi-
nates at one point and contin-
ues in one direction toward
infinity. A ray is defined as
part of a line on one side of a
point, and includes that point. Two letters are needed to name a ray. For example,
Ray ABis defined by point A,where it begins, and point B,the point the ray goes
through. (But note: Ray ABis not the same as ray BA.) If the initial point is not
included, the resulting figure is called a half line. In geometry, a ray is usually con-
sidered a half-infinite line with one of the two points Aand Btaken to be at infinity.What does the word parallelmean in geometry?
In geometry, parallel means two lines in two-dimensional Euclidean space that do not
172 intersect; or two lines in the same plane that never meet and that maintain the same
Parallel lines are found all around us in life, such as in the way we
farm land to grow crops in neat rows. National Geographic/Getty
Images.