Points of intersection of polar equations

In summary, by setting $\cos\left({2\theta}\right)=\cos\left({\theta}\right)$, we get the solutions $\theta=0,\frac{2\pi}{3},\frac{4\pi}{3}$ for the points of intersection of $\rho=\cos\left({2\theta}\right)$ and $\rho=\cos\left({\theta}\right)$. However, these solutions do not include all the intersection points as there are also points at the poles, such as $(0, \frac{\pi}{3})$ and $(0, \frac{\pi}{2})$. The reason for this is that the equation $\cos\left({2\theta}\
  • #1
Dethrone
717
0
Find the points of intersection of $\rho=\cos\left({2\theta}\right)$ and $\rho=\cos\left({\theta}\right)$

By setting $\cos\left({2\theta}\right)=\cos\left({\theta}\right)$, we get the solutions $\theta=0,\frac{2\pi}{3},\frac{4\pi}{3}$.

My question is how come that doesn't give us all the points of intersection? There are some points of intersection at the poles that were not obtained above, such as $(0, \frac{\pi}{3})$ and $(0, \frac{\pi}{2})$
 
Physics news on Phys.org
  • #2
Rido12 said:
Find the points of intersection of $\rho=\cos\left({2\theta}\right)$ and $\rho=\cos\left({\theta}\right)$

By setting $\cos\left({2\theta}\right)=\cos\left({\theta}\right)$, we get the solutions $\theta=0,\frac{2\pi}{3},\frac{4\pi}{3}$.

That's not quite the solution! :eek:

Properly solving it gives:
$$2 \theta = \pm \theta + 2 k \pi
\quad\Rightarrow\quad \theta = 2 k \pi \vee \theta = \frac 2 3 k \pi
\quad\Rightarrow\quad \theta = \frac 2 3 k \pi$$
where $k$ is any integer.
My question is how come that doesn't give us all the points of intersection? There are some points of intersection at the poles that were not obtained above, such as $(0, \frac{\pi}{3})$ and $(0, \frac{\pi}{2})$

Huh?
How did you get vectors? :confused:

Do you mean that $\theta=\frac \pi 2$?
If so, that is not a solution!

Or do you mean $y=\frac \pi 2$?
That makes even less sense! (Wasntme)
 
  • #3
I'm getting this straight from an example given by the textbook.

"Setting $\cos\left({2\theta}\right)=\cos\left({\theta}\right)$, then $\theta=0$, $\frac{2\pi}{3}$, $\frac{4\pi}{3}$, yielding points of intersection $(1,0)$, $(-\frac{1}{2}, \frac{2\pi}{3})$, and $(-\frac{1}{2}, \frac{4\pi}{3})$. But the pole is also an intersection point, appearing as $(0, \frac{\pi}{4})$ on $\rho=\cos\left({2\theta}\right)$ and as $(0, \frac{\pi}{2})$ on $\rho=\cos\left({\theta}\right)$"

I don't think they're using bracket notation to represent vectors, but rather $(\rho, \theta)$, where $\rho$ is the radius.

I like Serena said:
Do you mean that $\theta=\frac \pi 2$?
If so, that is not a solution!

My point and question exactly. $\theta=\frac \pi 2$ is indeed not a solution to $\cos\left({2\theta}\right)=\cos\left({\theta}\right)$, but it is a point of intersection, or at least the book says it is. My question again: Why are there points of intersection other than the ones that satisfy the above equation. Those points of intersections also happen to be at the origin/pole.
 
Last edited:
  • #4
Rido12 said:
Find the points of intersection of $\rho=\cos\left({2\theta}\right)$ and $\rho=\cos\left({\theta}\right)$

By setting $\cos\left({2\theta}\right)=\cos\left({\theta}\right)$, we get the solutions $\theta=0,\frac{2\pi}{3},\frac{4\pi}{3}$.

My question is how come that doesn't give us all the points of intersection? There are some points of intersection at the poles that were not obtained above, such as $(0, \frac{\pi}{3})$ and $(0, \frac{\pi}{2})$

Just putting in my two cents as I found your original solution to this equation very sloppy...

$\displaystyle \begin{align*} \cos{ \left( 2\theta \right) } &= \cos{ \left( \theta \right) } \\ 2\cos^2{ \left( \theta \right) } - 1 &= \cos{ \left( \theta \right) } \\ 2\cos^2{ \left( \theta \right) } - \cos{ \left( \theta \right) } - 1 &= 0 \\ 2\cos^2{\left( \theta \right) } - 2\cos{ \left( \theta \right) } + \cos{ \left( \theta \right) } - 1 &= 0 \\ 2\cos{ \left( \theta \right) } \left[ \cos{ \left( \theta \right) } - 1 \right] + 1 \left[ \cos{ \left( \theta \right) } - 1 \right] &= 0 \\ \left[ \cos{ \left( \theta \right) } - 1 \right] \left[ 2\cos{ \left( \theta \right) } + 1 \right] &= 0 \\ \cos{ \left( \theta \right) } - 1 &= 0 \textrm{ or } 2\cos{ \left( \theta \right) } + 1 = 0 \\ \cos{ \left( \theta \right) } &= 1 \textrm{ or } \cos{ \left( \theta \right) } = -\frac{1}{2} \\ \theta &= 2\pi\,n \textrm{ or }\theta = \left\{ \frac{2\pi}{3} , \frac{4\pi}{3} \right\} + 2\pi\,n \textrm{ where } n \in \mathbf{Z} \\ \theta &= \left\{ 0 , \frac{2\pi}{3} , \frac{4\pi}{3} \right\} + 2\pi n \end{align*}$
 
  • #5
Yup, that's what ILS said too and it makes sense. The solution I have here was given by my workbook, "Schaum's Outlines". I think the author was assuming the domain $[0, 2\pi]$ which is consistent with the other questions in the section. Though, I still don't understand why solving the equation doesn't give us all the points of intersection, such as those at the poles.
 
  • #6
Rido12 said:
Yup, that's what ILS said too and it makes sense. The solution I have here was given by my workbook, "Schaum's Outlines". I think the author was assuming the domain $[0, 2\pi]$ which is consistent with the other questions in the section.

With the domain $[0, 2\pi]$, $\theta=2\pi$ is also a solution. (Angel)
But yeah, a domain of $[0, 2\pi)$ is assumed somewhere.
Note that this should be specified somewhere in the book. Typically such assumptions are written at the beginning of the chapter or some such.
Though, I still don't understand why solving the equation doesn't give us all the points of intersection, such as those at the poles.

Nor do I.
However, it does look a bit odd to have $\rho = \cos \theta$.

It appears we're talking about polar coordinates.
This should be something like $x = \rho \cos \theta$ and $x = \rho \cos 2 \theta$.
Then we have a solution if $\cos\theta=\cos 2\theta$ or if $\rho = 0$.
 
  • #7
I'm working on a chapter dedicated to polar coordinates, and they such formulas:

$$\tan\left({\psi}\right)=\frac{\rho}{\rho '}$$

any ideas what they call this? $\psi$ is the angle between the radius vector OP and the tangent at P.

also,

$$\tan\left({\phi}\right)=\frac{\tan\left({\psi_1}\right)-\tan\left({\psi_2}\right)}{1+\tan\left({\psi_1}\right)\tan\left({\varPsi_2}\right)}$$

which is the angle of intersection.

So they use $\rho$ like that a lot.

In this case, we have solutions $\cos\theta=\cos 2\theta$ and when $\rho = 0$ too. Why doesn't $\cos\theta=\cos 2\theta$ cover all the points of intersection? Why do we also have to consider when $\rho=0$?
 
  • #8
Rido12 said:
I'm working on a chapter dedicated to polar coordinates, and they such formulas:

$$\tan\left({\psi}\right)=\frac{\rho}{\rho '}$$

any ideas what they call this? $\psi$ is the angle between the radius vector OP and the tangent at P.

What kind of name are you looking for? (Wondering)

Edit: I guess polar tangential angle will do.
Or the angle between the radial line and the tangent.
also,

$$\tan\left({\phi}\right)=\frac{\tan\left({\psi_1}\right)-\tan\left({\psi_2}\right)}{1+\tan\left({\psi_1}\right)\tan\left({\varPsi_2}\right)}$$

which is the angle of intersection.

That's the difference formula for the tangent:
$$\tan(\alpha \pm \beta) = \frac{\tan \alpha \pm \tan \beta}{1 \mp \tan \alpha \tan \beta}$$

So basically it says:
$$\phi = \psi_1 - \psi_2$$
So they use $\rho$ like that a lot.

In this case, we have solutions $\cos\theta=\cos 2\theta$ and when $\rho = 0$ too. Why doesn't $\cos\theta=\cos 2\theta$ cover all the points of intersection? Why do we also have to consider when $\rho=0$?

Any point with $\rho=0$ is the origin.
This is independent of the angle.

In polar coordinates we can write a point as $(\rho,\theta) = (4,\frac\pi 3)$, meaning it is the cartesian point $(x,y)=(2,2\sqrt 3)$.

Note that the points $(\rho,\theta)=(0,\frac\pi 3)$ and $(\rho,\theta)=(0,\frac\pi 6)$ are the same point.
They are simply the origin $(x,y)=(0,0)$. (Nerd)
 
  • #9
That makes sense now. They're are both the origin point... (Rofl)...but disguised (Smoking)
 

FAQ: Points of intersection of polar equations

What are "points of intersection" in polar equations?

Points of intersection refer to the coordinates where two or more polar equations intersect on a graph. These points represent the solutions to the system of equations and can be found by solving for the values of theta that satisfy both equations.

How can I determine the number of points of intersection in a polar equation?

The number of points of intersection in a polar equation depends on the degree and complexity of the equations. In general, two polar equations can have a maximum of two points of intersection, while three or more equations can have more than two points of intersection.

Can polar equations have more than one point of intersection?

Yes, polar equations can have multiple points of intersection. This occurs when the equations have multiple values of theta that satisfy both equations. These points of intersection can be found by solving for theta and plugging the values back into the equations to find the corresponding coordinates.

How do I graph points of intersection in polar coordinates?

To graph points of intersection in polar coordinates, first plot the individual equations on a polar coordinate plane. Then, mark the points where the equations intersect. These points will have both a theta and radius value, which can be plotted on the graph as a coordinate.

Are there any special cases when working with points of intersection in polar equations?

Yes, there are a few special cases to keep in mind when working with points of intersection in polar equations. These include when one equation is a linear function and the other is a quadratic function, when the equations are symmetric about the polar axis, and when the equations have a common factor. In these cases, the number of points of intersection may be limited or there may be no solutions at all.

Similar threads

Replies
8
Views
1K
Replies
29
Views
2K
Replies
3
Views
2K
Replies
3
Views
968
Replies
3
Views
2K
Replies
15
Views
3K
Back
Top