Equations of lines

There are many ways of representing a mathematical line. You are probably most familiar with point-y-intercept form,


That equation is useful when you know the slope and the y-intercept. Also, that form is easy to express in function notation, like f(x)=mx+b, and it fits nicely with the other families of functions. Another way of describing a line is with standard form:


This form gives you both the intercepts, but most of the time it is inconvenient. Yet another type of line equation is point-slope form:


Not surprisingly, this is useful when you know a point and the slope.

Those three equations have something in common: they relate x to y. If you solve for y (already done for you in point-y-intercept form), you can choose an x-value and the equation will tell you the y-value. This isn’t the only way of doing it, as we will see. In particular, it doesn’t scale well to three-space; our new method will.

We are going to explore another triplet of line equations, starting with the vector equation of the line. To describe a line with vectors, we need an initial point that falls on the line—we’ll call it point A. From point A, we need a vector that tells us which way the line goes—we’ll call this m. Here’s what this might look like:

A line defined by an initial point and a direction vector

To get from point A to some point P elsewhere on the line, all we have to do is add a scalar multiple of the direction vector:


The value of t controls how far along the line we go from A. If it is negative, we go in the opposite direction. Instead of using origin-to-point notation, we usually use r, which is implied to be a position vector (the r stands for radius, and a position vector is a radius from the origin). Here is the conventional way of writing the vector equation of the line:

r=r0+tm, t.

Suppose we substitute for vectors in 2 with variables as components:

[x,y]=[x0,y0]+t[a,b], t.

Breaking this up into two equations gives us the parametric equations of a line (two equations because there are two components):

x=x0+ta and y=y0+tb, t.

There is an important difference between this form and the earlier ones. The other equations relate x and y such that we can choose one an obtain the other. With the parametric equations, we choose a value for the free parameter t. We plug this into the equations and they tell us x and y. Instead of x being related to y, they are both independently controlled by t.

If we solve the parametric equations for t, we get the symmetric form:


This form makes it easy to test if a particular point falls on the line: just evaluate both sides and see if they produce the same t-value.