Equation of a Circle (Cambridge (CIE) A Level Maths) : Revision Note
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Equation of a Circle
What do I need to know about the equation of a circle?
A circle with centre (a, b) and radius r has the equation

You need to be able to find the centre and radius of a circle from its equation

You need to be able to find the equation of a circle given its centre and radius

Examiner Tips and Tricks
Remember that the numbers in the brackets have the opposite signs to the coordinates of the centre.
Don't forget to take the square root of the right-hand side of the equation when finding the radius.
Worked Example

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