# How do I enter circle equations in a Function Definition?

I am trying to use Function Definitions to enter a circle eg (x – a)^2 + (y – b)^2 = r^2 so that I can then use the constant controller to vary a, b and r. How do I do this? If I enter the above equation I get the wrong graph. If I try to re-arrange the equation to get it in terms of y then I get a square root involved and then I only get half of the circle shown (as the positive sqrt solution only is used). Also there doesn't seem to be an equation editor for entering function definitions. Sorry lots of questions I know!

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