The purpose of this post is to briefly discuss parallelism and perpendicularity of lines in both a geometric and algebraic setting.
Lines
What is a line? In Euclidean Geometry we usually don’t define a line and instead call it a primitive object (the properties of lines are then determined by the axioms which refer to them). If instead points and line segments – defined by pairs of points
–
are taken as the primitive objects, the following might define lines:
Geometric Definition Candidate
A line,
, is a set of points with the property that for each pair of points in the line,
,
.
In terms of a picture this just says that when you have a line, that if you take two points in the line (the language in comes from set theory), that the line segment is a subset of the line:

Exercise:
Why is this objectively not a good definition of a line.
Once we move into Cartesian\Coordinate Geometry we can perhaps do a similar trick. We can use line segments, and their lengths to define slope, (slope = rise over run) and then define a line as follows:
Algebraic Definition Candidate
A line,
, is a set of points such that for all pairs of distinct points
, the slope is a constant.
This means that if you take two pairs of distinct points in a line
, and then calculate the slopes between them, you get the same answer, and therefore it makes sense to talk about the slope of a line,
.

This definition, however, has exactly the same problem as the previous. The definition we use isn’t too important but I do want to use a definition that considers the line a set of points.
The Equation of a Line
We can use such a definition to derive the equation of a line ‘formula’ for a line of slope
containing a point
.
Suppose first of all that we have an
axis and a point
in the line. What does it take for a second point
to be in the line?

Read the rest of this entry »
Recent Comments