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# 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

x2 + y2 = 1                                (1)

### Standard Form

The standard form for the equation of a circle can be expressed as

(x - a)2 + (y - b)2 = r2                            (2)

where

a = x coordinate position of the circle center

b = y coordinate position of the circle center

### General Form

The general form for the equation of a circle can be expressed as

x2 + y2 + A x + B y + C = 0                                  (3)

where

A, B, C = constants

r = (a2 + b2 - C) 1/2                                       (4)

### Coordinates in the Circle Centre

a = - A / 2                                   (5a)

b = - B / 2                                   (5b)

## Related Topics

• Mathematics - Mathematical rules and laws - numbers, areas, volumes, exponents, trigonometric functions and more

## Tag Search

• en: circle equation
• es: círculo ecuación
• de: Kreisgleichung

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## Citation

• Engineering ToolBox, (2014). Circle - Equation. [online] Available at: https://www.engineeringtoolbox.com/circle-equation-d_1904.html [Accessed Day Mo. Year].

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