A-Math - Circles - Find the centre and equation of circle.

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There are two methods of finding the center and radius of circle. The first method is the formula method, the second is completing the squares method. I advice you to use the completing the squares method because students often forget the formula.

Factorize out any coefficient of xand y2 before you carry out completing the squares.  Then rearrange the equation to the standard form (x-a)2+(y-b)2=r2. format, where (a,b) is the coordinates of the center of circle and r is the radius.


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