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So far, the graphs we have drawn are defined by one equation: a function with
two variables,
*x*
and
*y*
. In some cases, though, it is useful to introduce a
third variable, called a parameter, and express
*x*
and
*y*
in terms of the
parameter. This results in two equations, called parametric equations.

Let
*f*
and
*g*
be continuous functions
(functions whose graphs are unbroken curves) of the variable
*t*
. Let
*f* (*t*) = *x*
and
*g*(*t*) = *y*
. These equations are parametric equations,
*t*
is the
parameter, and the points
(*f* (*t*), *g*(*t*))
make up a plane curve. The
parameter
*t*
must be restricted to a certain interval over which the functions
*f*
and
*g*
are defined.

The parameter can have positive and negative values. Usually a plane curve is
drawn as the value of the parameter increases. The direction of the plane curve
as the parameter increases is called the orientation of the curve. The
orientation of a plane curve can be represented by arrows drawn along the curve.
Examine the graph below. It is defined by the parametric equations
*x* = cos(*t*)
,
*y* = sin(*t*)
,
0≤*t* < 2*Π*
.

Figure %: A plane curve defined by the parametric equations
*x* = cos(*t*)
,
*y* = sin(*t*)
,
0 < *t*≤2*Π*
.

The unit circle is an example of a curve that can be easily drawn using parametric equations. One of the advantages of parametric equations is that they can be used to graph curves that are not functions, like the unit circle.

Another advantage of parametric equations is that the parameter can be used to
represent something useful and therefore provide us with additional information
about the graph. Often a plane curve is used to trace the motion of an object
over a certain interval of time. Let's say that the position of a particle is
given by the equations from above,
*x* = cos(*t*)
,
*y* = sin(*t*)
,
0 < *t*≤2*Π*
, where
*t*
is time in seconds. The initial position of the particle (when
*t* = 0
)is
(cos(0), sin(0)) = (1, 0)
. By plugging in the number of seconds
for
*t*
, the position of the particle can be found at any time between
0
and
2*Π*
seconds. Information like this could not be found if all that was known
was the rectangular equation for the path of the particle,
*x*
^{2} + *y*
^{2} = 1
.

It is useful to be able to convert between rectangular equations and parametric equations. Converting from rectangular to parametric can be complicated, and requires some creativity. Here we'll discuss how to convert from parametric to rectangular equations.

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