# Conics in polar equation form

1. Apr 30, 2005

I think that i'm over looking something with this problem. Below is the equation of an hyperbola in polar form.

$$R=\frac{1}{1 + 2cos{\theta}}$$

when $$\theta =\pi$$ shouldn't $$R = -1$$? And not $$R= 1$$
Am I over looking some property of the $$\cos$$ function?
Even when i evalute this expression at $$\theta=\pi$$in my ti-89 i get that R is = to 1. What am i not seeing?

2. Apr 30, 2005

### bahamut

You have made a mistake,im afraid.
Even if you find that R=-1,it is just a mathmatic form.
R is always positive,you can just say R=|1/2cos(theta)|
If still don't understand,contact me at wangkehandsome@hotmail.com,I will be glad to anwser it for you and even be more glad if you point out my fallacy.

3. Apr 30, 2005

### Data

Well, then he hasn't made a mistake: the given expression is just wrong!

4. Apr 30, 2005

$$(r,\theta)$$ coordinate system are $$(1,\pi)$$ because the radius is always positive