In plane Euclidean geometry, a rhombus (plural rhombi or rhombuses) is a simple (non-self-intersecting) quadrilateral whose four sides all have the same length. Another name is equilateral quadrilateral, since equilateral means that all of its sides are equal in length. The rhombus is often called a diamond, after the diamonds suit in playing cards.

Sides of parallelogram: a,b

Diagonals: d1,d2

Consecutive angles: α,β

Angle between the diagonals: φ

Altitude: h

Perimeter: L

Area: S

- $α+β=180°$
- $s_1^2+d_2^2=4a^2$
- $h=a\ sin\ α={d_1d_2}/{2a}$
- $r=h/2={d_1d_2}/{4a}={a\ sin\ α}/2$
- $L=4a$
- $S=ah=a^2\ sin\ α$

$S=1/2d_1d_2$

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