Circle
	All points on the circumference of a circle are equidistant from its center. Radius: r Diameter: d Circumference: C Area: K
		d = 2r C = 2 p r = p d   K = p r2 = p d2/4 C = 2 sqrt(p K)   K = C2/4 p = Cr/2

Arc of a Circle
	A curved portion of a circle. Length: s   Central angle:      theta (in radians),      alpha (in degrees)       s = r theta = r alpha p/180

Segment of a Circle
	Either of the two regions into which a secant or a chord cuts a circle. Chord length: c Height: h Distance from center of circle to chord's midpoint: d Central angle: theta (in radians), alpha (in degrees)   Area: K Arc length: s
	theta = 2 arccos(d/r) = 2 arctan(c/(2d)) = 2 arcsin(c/(2r))      h = r - d      c = 2 sqrt(r2-d2) = 2r sin(theta/2) = 2d tan(theta/2) = 2 sqrt[h(2r-h)]      d = sqrt(4r2-c2)/2 = r cos(theta/2) = c cot(theta/2)/2      K = r2[theta-sin(theta)]/2 = r2arccos([r-h]/r) - (r-h)sqrt(2rh-h2)          = r2arccos(d/r) - d sqrt(r2-d2)      theta = s/r      K = r2[s/r - sin(s/r)]/2

Sector of a Circle
	The pie-shaped piece of a circle 'cut out' by two radii. Central angle:      theta (in radians),      alpha (in degrees)   Area: K Arc length: s      K = r2theta/2 = r2alpha p/360      theta = s/r      K = rs/2

Equation of Circle: (Cartesian coordinates)

  for a circle with center (j, k) and radius (r):
    (x-j)² + (y-k)² = r²

Equation of Circle: (polar coordinates)
    for a circle with center (0, 0):   r(u) = radius

    for a circle with center with polar coordinates: (c, a) and radius a:
      r² - 2cr cos(u - a) + c² = a²

Equation of a Circle: (parametric coordinates)
    for a circle with origin (j, k) and radius r:
      x(t) = r cos(t) + j       y(t) = r sin(t) + k