Quadrangles, polygons

Sides and diagonals of a parallelogram
2(a^2+b^2) = d1^2 + d2^2

Sides and diagonals of a rhombus
d_1^2 + d_2^2 = 4a^2

Diagonal of a square
d = a saknis(2)

Trapezoid midline
m = (a+b) / 2

Height of an cyclic (inscribed ) antiparallelogram (isosceles trapezoid)
h = saknis(a*b)

The inradius of quadrilateral
r = S/p

Angle of a regular polygon
α = (n - 2) / n * 180

Sum of the angles of a polygon
s = (n - 2) * 180

Side of a regular polygon
a = 2 R sin(180 / n)

Inradius of a regular polygon
r =R cos(180 / n)

Circumradius of a square
R = a saknis(2) / 2

Inradius of a square
r = a / 2

Circumradius of a regular hexagon
R = a

Inradius of a regular hexagon
r = a saknis(3) / 2