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Formula for geometrical figures

pi (π)=3.1415926535 ...

Perimeter formula
ShapeFormula
Circle2 × pi × radius
Square4 × side
Rectangle2 × (length + width)
Parallelogram2 × (side1 + side2)
Triangleside1 + side2 + side3
Regular n-polygonn × side
Trapezoidheight × (base1 + base2) / 2
Trapezoidbase1 + base2 + height × [csc(theta1) + csc(theta2)]
Ellipse4 × radius1 × E(k,pi/2)
E(k,pi/2) is the complete elliptic integral of the second kind
k = (1/radius1) × sqrt(radius12 - radius22)

Source: Spiegel, Murray R. Mathematical Handbook of Formulas and Tables.
Schaum's Outline series in Mathematics. McGraw-Hill Book Co., 1968.