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**Lepzed** I've been given 3 coordinates: (0,0), (2,0) and (0,2) and I'm asked to draw the circle and then determine the equation of the circle: $\displaystyle (m - x)^2 + (n - y)^2 = r^2$ .

Now, since r=2 and the center point is (0,0), I'd say the equation would be:

$\displaystyle x^2 + y^2 = 4$, my book tells me it should be $\displaystyle x^2 + y^2 - 2x - 2y = 4$ (or something like that, don't have it with me). Could someone elaborate on this? I don't get where the -2x,-2y come from!