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Mathematical Expression Editor
Parametric Equations of Lines
At this point you should be very familiar with graphing linear equations of the form ,
where is the slope of the line and is the -intercept. Unfortunately, there is no
equivalent way of representing lines in , or better yet, . In this section we will develop
an alternative way of representing lines.
Parametric Lines in
Imagine a ladybug crawling around a plane. At every instant in time, the ladybug’s
position in the plane can be described by an ordered pair . Coordinates and are
functions of time. Suppose the position of the ladybug at time is given by:
Our goal is to sketch the path of the ladybug during the time interval . To do this we
will make a table of values, just like you did when you first started sketching graphs
of functions.
The point , corresponding to each value of is the location of the ladybug in the
coordinate plane at time .
It appears that the points corresponding to lie on a line with slope . In the next
problem we will find the equation of the line that contains the path of the
ladybug.
In Exploration init:paramline2d we considered equations
that described the path of a ladybug in the plane. We will now consider these
equations in a broader context as simply describing a curve in the plane. We
conjectured that the curve described by these equations is a line with slope . We will
now find the equation of this line.
One approach to finding the equation is to solve one of the given equations for , then
substitute into the other equation. Solving for gives us
Substituting this expression into we get
Plotting the line together with the path we plotted earlier, we see that our original
path lies on the line.
If we do not restrict to values between 0 and 3, we can get every point on the line
.
Equations such as
are called parametric equations, and is called a parameter.
When given an equation of the form , we recognize it as an equation whose graph is a
line and we don’t need to make a table of values to sketch the graph of the equation.
We should be able to do the same for parametric equations of lines. In the next
Exploration we will examine our equations carefully to see if we can discern
any patterns that would help us plot the line without making a table of
values.
Let’s return to parametric equations from the previous problem.
Consider the table of values we constructed in Exploration init:paramline2d.
Observe that every time increases by , increases by and decreases by
. An increase by in the -coordinate combined with a decrease of in the
-coordinate corresponds to the coefficients and of in the parametric equations.
Our findings are in agreement with the fact that the slope of the line is
.
We will capture the “rise” and “run” aspect of the line by using vector .
Sketching together with the line, we find that the vector is parallel to the
line.
Vector is called a direction vector. Observe that the components of are the
coefficients of in the parametric equations.
Next, let’s turn our attention to constants and . They are the values of and when ,
and correspond to the point .
Recall that in Exploration init:paramline2d we said that the given parametric equations describe the
position of a ladybug crawling in the coordinate plane. If we are concerned with the
position of the ladybug at time , the point is very important - this is where the bug
is located when . But, if we are using parametric equations simply to describe the line
, without regard for when the bug is located at each point, then the point is not any
more special than any other point on the line. In fact, we can use any other point
on this line to find another set of parametric equations that describe the
same line. In Practice Problem prob:paramnotunique you will be asked to show that equations
describe the same line. Thus, parametric representations are not unique.
Let be a direction vector for line , and let be an arbitrary point on . Then the
following parametric equations describe :
Find parametric equations for line if contains the point and is parallel to line with
parametric equations
To find a set of parametric equations for , we need a point on and a direction vector.
Point is on . Line is parallel to , so we can use , a direction vector for , as a
direction vector for . This gives us the following parametric equations for :
Parametric Lines in
Consider the following set of parametric equations
We will begin by making a table of values.
Observe that every time increases by , increases by , increases by , and increases
by . The pattern “out , over , up ” is illustrated in the diagram.
We will use a direction vector to represent the direction of the line. Note that
the components of are the same as the coefficients of in the parametric
equations.
Also, observe that the constants correspond to the coordinates of the point on the
line for which .
As before, the point may be replaced with any other point on the line to produce a
different set of parametric equations describing the same line.
We can generalize our observations in Exploration init:paramline3d as follows.
Let be a direction vector for line , and let be an arbitrary point on . Then the
following parametric equations describe :
Find a set of parametric equations for a line in that passes through and .
We need
a point and a direction vector. To find the direction vector we will use the “head-tail”
formula (Formula form:headminustailrn) to find the vector whose tail is at and whose head is at . The
direction vector is
For our point, we can pick either of the two given points. The equations will differ
depending on the point we pick, but they will describe the same line. Remember,
parametric representations are not unique! If we choose for our point we get the
following set of parametric equations:
Parametric Lines in
After working with parametric equations of lines in and , it should be easy to
surmise what parametric equations of lines look like for lines in .
Let be a direction
vector for line in , and let be an arbitrary point on . Then the following parametric
equations describe :
From Parametric Equations to Vector Equations
Consider again the parametric equations from Exploration init:paramline2d.
These equations describe the line by providing coordinates of individual points on the
line, as shown in the left figure below.
If we consider the points on the line to be heads of vectors, then we can think of the
line as being “traced” by a vector as changes (See above, right). We can capture this
algebraically by using a vector equation for the line:
Note that the direction vector and the point , now vector , are easy to identify in
the equation.
We can use vector equations to describe lines in using the following formula.
Let be a direction vector for line in , and let be an arbitrary point on . Then the
following vector equation describes :