# Polar conics

1. Nov 14, 2013

### Jbreezy

1. The problem statement, all variables and given/known data
I want to know if I did these right.

Write a polar equation of a conic with the focus at the origin and the given data.
2. Relevant equations

r = (ed) /(1+- cos(theta)) and r = r = (ed) /(1+- sin(theta))

3. The attempt at a solution

Parabola , directrix x = -3 I came up with r = -3/ (1 -cos(θ))

Hyperbola, eccentricity 3, directrix x = 3
I got 9/(1+3cos(θ))

What do you think OK?

2. Nov 15, 2013

### Simon Bridge

Did you try plotting them?
For instance, a parabola with directorix -3 ... what would a sketch of it look like?
Does this match up with some values - like what would r be at zero angle?
Does that match up with your equation?

3. Nov 15, 2013

### Jbreezy

Yeah I plotted them. Idk tho

4. Nov 15, 2013

### Simon Bridge

And? .... what did you find out?

Idk <looks it up> "I don't know"?
What don't you know?

Tho <looks it up>
1. From Middle English tho, tha, from Old English þā (“the, those”, plural), from Proto-Germanic *þai (“those”), from Proto-Indo-European *to-, *só (“that”).
2. abbv. Thor Industries
3. Internet slang - short for "though"
<sigh>

If you were to sketch a parabola with a directorix at x=3,
- is the parabola oriented with the +y axis? The +x axis? something else?
- where is the focus?
- what sort of value should r have at easy angles like 0, 90, 180, 360?
- will r ever be negative?

How does this compare with the results fro your equation: $r=-3/(1-\cos\theta )$ ?

If you don't know the basic properties of a parabola, without using the equation, then you need to learn them.
http://en.wikipedia.org/wiki/Conic_section

Last edited: Nov 15, 2013
5. Nov 17, 2013

### Jbreezy

Hyperbola, eccentricity 3, directrix x = 3
I got 9/(1+3cos(θ)) This one is to the right. The some. The question say's " Write a polar eq. of a conic with the focus at the origin and the given data".
So, this one OK?

6. Nov 18, 2013

### Simon Bridge

Well ... write down a similar list to before and check it off.
What do you get?

7. Nov 18, 2013

### Simon Bridge

Well ... write down a similar list to before and check it off.
What do you get?