Circle  Equation
The equation for a circle
Unit Circle
The equation of a unit circle  where the center of the circle is located in the origo of the coordinate system and the radius = 1  can be expressed as
x^{2} + y^{2} = 1 (1)
Standard Form
The standard form for the equation of a circle can be expressed as
(x  a)^{2} + (y  b)^{2} = r^{2} (2)
where
a = x coordinate position of the circle center
b = y coordinate position of the circle center
r = circle radius
General Form
The general form for the equation of a circle can be expressed as
x^{2} + y^{2} + A x + B y + C = 0 (3)
where
A, B, C = constants
Radius of circle
r = (a^{2} + b^{2}  C) ^{1/2} (4)
Coordinates in the Circle Centre
a =  A / 2 (5a)
b =  B / 2 (5b)
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