Find an equation of the circle with centre at (0,0) and radius r.

Explanation

To find the equation of a circle with its center at (0, 0) and radius \( r \), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the General Equation of a Circle**:
The general equation of a circle with center at \((h, k)\) and radius \( r \) is given by:
\[
(x - h)^2 + (y - k)^2 = r^2
...