The Lesson
The equation of a circle, in general form is in the form:
- x and y are the Cartesian coordinates of points on the (boundary of the) circle.
- A, B and C are constants. They have no meaning.

A Real Example of an Equation of a Circle (in General Form)
An example of an equation of a circle in general form is given below:
x2 + y2 − 2x − 4x − 4 = 0
(x − 1)2 + (y − 2)2 = 9
The Signs in the Equation
The equation of a circle in general form is:
x2 + y2 + Ax + By + C = 0
A, B and C can be positive or negative or even 0.
For example,
- A = 1, B = −2 and C = 3
- A = −4, B = 0 and C = −5
x2 − y2 + x + 2y − 3 = 0
...it isn't an equation of a circle.