# Homework Help: Equation of a circle / polar coordinates

1. Dec 4, 2011

### Marioqwe

I was looking at the equation of a circle in polar coordinates on wikipedia,

http://en.wikipedia.org/wiki/Polar_coordinate_system

and I understand that a is the radius of the circle, and that (r0, phi) is the center of the circle. But I don't see what the r and theta refer to :(.

2. Dec 4, 2011

### eumyang

r and θ are just the variables in the equation.

Just like in rectangular coordinates, given a center (h, k) and radius r,
(x - h)2 + (y - k)2 = r2
is the equation of the circle, and x and y are the variables.

Or am I misunderstanding your question?

3. Dec 4, 2011

### Marioqwe

So do they just represent a point inside the circle?

4. Dec 4, 2011

### eumyang

No, they represent a set of points that lie on the circle, not inside.

5. Dec 4, 2011

### Marioqwe

What do you mean by [[/B]on[/B]? And by the way; sorry for giving you a hard time. English is not my first language.

6. Dec 4, 2011

### eumyang

See attached picture. The red point is on the circle, while the blue point is inside the circle.

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