Useful information about circles:

Standard Form:		(x-h)²+(y-k)²=r²
Eccentricity:		e=0
Center:			(h,k)
Diameter:		2r
Parametric Form:	x′=rsin(t) or	x′=rcos(t)
			y′=rcos(t) 	y′=rsin(t)

See also: