# Math Help - Polar graph: stuck on sketching a cardioid

1. ## Polar graph: stuck on sketching a cardioid, something about chords

Sketch the cardioid with polar equation r=2(1+cos theta)
Prove that for all the chords POQ of the cardioid which pass through the pole O, the length of PQ is constant.
How do I do the green part?
I have gone as far as to deduce that if P (r1, theta1) and Q is (r2, theta2)
Then theta2=theta1-180
r2=2+2cos (theta1-180)
There isn't given a formula in the book to find the distance between two polar coordinates, I found one on net:

Here is the sketch of the graph:

I am stuck here for over two hours. Emergency help...I am drow...(Oops I gulped up some water)

There isn't given a formula in the book to find the distance between two polar coordinates, I found one on net:
You don't need any formula from the internet... The distance from $O$ to $P(r,\theta)$ is $r$. So what you have to do is add the values of $r$ for $\theta_1$ and $\theta_2=180-\theta_1$ (in degrees), and check that it does not depend on $\theta_1$.

3. Originally Posted by Laurent
You don't need any formula from the internet... The distance from $O$ to $P(r,\theta)$ is $r$. So what you have to do is add the values of $r$ for $\theta_1$ and $\theta_2=180-\theta_1$ (in degrees), and check that it does not depend on $\theta_1$.
theta2=theta1-180 according to my calculations. I am giving it a try.

theta2=theta1-180 according to my calculations. I am giving it a try.
Sorry I made a mistake, in fact $\theta_2=\theta_1+180$. Because $\theta_2-\theta_1$ is a flat angle.

5. Originally Posted by Laurent
You don't need any formula from the internet... The distance from $O$ to $P(r,\theta)$ is $r$. So what you have to do is add the values of $r$ for $\theta_1$ and $\theta_2=180-\theta_1$ (in degrees), and check that it does not depend on $\theta_1$.
It worked. Thanks.

6. Originally Posted by Laurent
Sorry I made a mistake, in fact $\theta_2=\theta_1+180$. Because $\theta_2-\theta_1$ is a flat angle.
Does that mean I am wrong here? I have verified it with calculator, my formula worked for polar -pi<theta<pi

No, indeed, you're right as well... I considered the graph defined for $0<\theta_2<2\pi$, while you considered $-\pi<\theta_2<\pi$, that's why. Forget my previous post, I was correcting my first one, not yours in fact.
No, indeed, you're right as well... I considered the graph defined for $0<\theta_2<2\pi$, while you considered $-\pi<\theta_2<\pi$, that's why. Forget my previous post, I was correcting my first one.