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)

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