Equilic Quadrilateral
A Quadrilateral in which a pair of opposite sides have the same length and are inclined at 60° to each other(or equivalently, satisfy 
). Some interesting theorems hold for such quadrilaterals. Let 
be an equilic quadrilateral with 
and . Then
- 1. The Midpoints
, 
, and 
of the diagonals and the side 
always determine anEquilateral Triangle. - 2. If Equilateral Triangle
is drawn outwardly on , then 
is also an EquilateralTriangle. - 3. If Equilateral Triangles are drawn on
, 
, and 
away from 
, thenthe three new Vertices , , and are Collinear.
References
Garfunkel, J. ``The Equilic Quadrilateral.'' Pi Mu Epsilon J. 7, 317-329, 1981. Honsberger, R. Mathematical Gems III. Washington, DC: Math. Assoc. Amer., pp. 32-35, 1985.
- Dedekind Cut
- Dedekind Eta Function
- Dedekind Function
- Dedekind Ring
- Dedekind's Axiom
- Dedekind's Problem
- Dedekind Sum
- Deducible
- Deep Theorem
- Defective Matrix
- Defective Number
- Deficiency
- Deficient Number
- Definable Set
- Defined
- Definite Integral
- Degenerate
- Degree
- Degree (Algebraic Surface)
- Degree (Map)
- Degree of Freedom
- Degree (Polynomial)
- Degree Sequence
- Degree (Vertex)
- Dehn Invariant