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How to give the equation of a circle

I'm doing EDEXCEL A level maths and for my C2 exam there's going to be at least 1 question where I have to work out the equation of a circle. Usually I will work it out and give my final answer as x^2+y^2-ax-by-c=0 but usually in exam mark schemes the answer is given as (x-a)^2 + (y-b)^2 = r^2 . I understand that the two are identical but just written differently but does it matter which one I write if they both man the same?

Thanks :smile:
I'd go for the second one.
They will want it in the form (x-a)^2 + (y-b)^2 = r^2

Do you know how to convert your form to the above?
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maxcharlish
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Yeah thanks. I usually work it out and get my answer in that form then change it to one polynomial
Original post by maxcharlish
I'm doing EDEXCEL A level maths and for my C2 exam there's going to be at least 1 question where I have to work out the equation of a circle. Usually I will work it out and give my final answer as x^2+y^2-ax-by-c=0 but usually in exam mark schemes the answer is given as (x-a)^2 + (y-b)^2 = r^2 . I understand that the two are identical but just written differently but does it matter which one I write if they both man the same?

Thanks :smile:


Unless the format is specified in the question it doesn't matter. You are wasting exam time by taking the standard format and expanding the brackets. As soon as they see the (x-a)^2 + (y-b)^2 = r^2 they will award the marks and ignore your subsequent working.


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