Shared16 - Parametric Equations Assignment / Parametric Equations Notes.sagewsOpen in CoCalc
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  • Intro to Sage
  • Graphing and Solving Equations
  • Tangent Lines

Parametric Equations

Suppose x and y are both functions of a variable t , called the "parameter." Then each value of t gives a point in the x-y plane, (x(t),y(t)) . The set of all such points as t varies is called a "parametric curve," and the equations defining this curve are called "parametric equations."

Example 1

Below is an example of a parametric curve. Notice that y is not a function of x (or vice versa). Graphs of functions form a really limited collection of curves, and parametric curves provide many more kinds of graphs.

%var t

Below is an animation which shows the above curve being drawn as t starts at 0 and increases to \pi .

%var t
for n in [1..50]:

You can graph a parametric curve by hand using a table of values - just choose some values of t and plug them into the x and y functions. This is usually pretty tedious.

Sage can handle parametric curves using the parametric_plot command, as in the example above.

First, declare the variable t . Then define x(t) and y(t) . Finally, type parametric_plot((x(t),y(t)),(t,0,pi)). Notice that (t,0,pi) controls which values of t are used. You may want to increase pi if the graph looks incomplete.

Example 2

There is a toy called the Spirograph that lets you draw interesting curves using a collection of wheels. We can produce these pictures using Sage.

In the interact below, experiment with different values of a and b . If the curve looks incomplete, then increase tmax.

For example, try a=21,\ a=\frac{1}{2},\ a=\sqrt{2} (increase tmax to 100*pi for this one).

def _(a=5,b=2,tmax=10*pi):
    %var t
Interact: please open in CoCalc

Tangents to Parametric Curves

We would like to do calculus with parametric curves, such as finding the slope of the curve.

Example 3

Consider the parametric curve below, which has equations x(t)=2\sin(2t) and y(t)=2\sin(t) .

Although y is not a function of x , it looks like the curve should have tangent lines. How do we find the slope of the tangent line?

%var t

By the Chain Rule: \displaystyle\frac{dy}{dt}=\frac{dy}{dx}\cdot\frac{dx}{dt} .

If \displaystyle\frac{dx}{dt}\ne 0 , then we can solve for \displaystyle\frac{dy}{dx} to get


In other words, the slope of the curve in the x-y plane is given by \displaystyle\frac{y\,'(t)}{x'(t)} .

Notice that this slope is given as a function of t . So if we want the slope of the curve at a particular point (x,y) , then we need to find a value of t that gives us this point.

Example 4

Find an equation for the tangent line to the curve above at t=\frac{\pi}{6} .

First, find the slope function. I'll call this function m .

%var t
\displaystyle \frac{\cos\left(t\right)}{2 \, \cos\left(2 \, t\right)}

Now let's find the slope when t=\frac{\pi}{6} .

\displaystyle \frac{1}{2} \, \sqrt{3}

Next, we calculate x\left(\frac{\pi}{6}\right) and y\left(\frac{\pi}{6}\right) , then we use the point-slope form of a line:


sqrt(3) 1

Notice that the tangent line is a function of x , not t . In order to not interfere with our parametric function x(t) , I will use capital X for the tangent line.

TL(X)=1+sqrt(3)/2*(X-sqrt(3))  #Note the capital X
\displaystyle \frac{1}{2} \, \sqrt{3} {\left(X - \sqrt{3}\right)} + 1

Intersection Points

What happens to the derivative when the curve crosses itself?

Example 5

In the curve above, the curve intersects itself at (0,0) .

What values of t result in (x(t),y(t))=(0,0) ?

We need a value of t that gives both x(t)=0 and y(t)=0 .

First, we'll ask Sage to solve the equations.

%var t
[t == 0] [t == 0]

Sage tells us that t=0 will work. Is that the only possiblity?

No, we know there are more solutions, since x and y are both periodic functions. We can get Sage to give us a more complete answer by adding the optional argument to_poly_solve='force' (don't worry about what this does).

[t == 1/2*pi*z45] [t == pi*z50]

In the output above, the variables z45 and z50 are assumed to be any integer (that's what the "z" is for).

In other words, x(t)=0 when t=\frac{z\pi}{2} for any integer z , i.e., t=0,\ \pm\frac{\pi}{2},\ \pm\frac{2\pi}{2}=\pm\pi,\ \pm\frac{3\pi}{2},\ \pm\frac{4\pi}{2}=\pm 2\pi, etc.

On the other hand, y(t)=0 when t=z\pi for any integer z , i.e. t=0,\ \pm\pi,\ \pm 2\pi,\ \pm3\pi, etc.

The values of t on both of these lists result in both x and y being 0 .

Look at the two lists, and see what they have in common. In this case, both lists have t=z\pi .

What is the slope of the curve when t=z\pi ? Let's try a few values of z .

m(-2*pi); m(-1*pi); m(0*pi); m(1*pi); m(2*pi)
1/2 -1/2 1/2 -1/2 1/2

We get two different slopes: \frac{1}{2} and -\frac{1}{2} .

Since there are two different slopes, there must be two different tangent lines.

TL1(X)=0+1/2*(X-0)  #Note the capital X
TL2(X)=0-1/2*(X-0)  #Again, capital X