2D GEOMETRY FORMULA
CIRCLE r = radius; d = diameter DIAMETER : d = 2r AREA : A = (pi) r^2 CIRCUMFERENCE : C = 2(pi)r = (pi)d SECTOR r = radius; angle(AOB) = angle in radius AREA : A = 1/2 (pi) r^2 ARC LENGTH : s = angle(AOB) r ELLIPSE a = semimajor axis b = semiminor axis AREA : A = (pi) ab CIRCUMFERENCE : C = (pi)[3(a+b) - sqrt{(a+3b)(b+3a)} ANNULUS (COCENTRIC CIRCLES) r = inner radius, R = outer radius AVERAGE RADIUS = 1/2(r+R) WIDTH : w = R-r AREA : A = pi(R^2 - r^2) or A = 2*pi*22/7*average radius*width REGULAR POLYGON s = side length, n = number of sides CIRCUMFERENCE : R = 1/2*s*cos(pi/n) AREA : A = 1/4*n*s^2*cot(pi/n) or A = 1/2*n*R^2*sin(2*pi/n)