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The rhombic Penrose tiling contains two types of rhombus, a thin rhombus with angles of and , and a thick rhombus with angles of and . All side lengths are equal, but the ratio of the length of sides to the short diagonal in the thin rhombus equals 1 : φ {\displaystyle 1\mathbin {:} \varphi } , as does the ...
Using congruent triangles, one can prove that the rhombus is symmetric across each of these diagonals. It follows that any rhombus has the following properties: Opposite angles of a rhombus have equal measure. The two diagonals of a rhombus are perpendicular; that is, a rhombus is an orthodiagonal quadrilateral. Its diagonals bisect opposite ...
The golden rhombus. In geometry, a golden rhombus is a rhombus whose diagonals are in the golden ratio: [1] = = + Equivalently, it is the Varignon parallelogram formed from the edge midpoints of a golden rectangle. [1]
The ratio of the long diagonal to the short diagonal of each face is exactly equal to the golden ratio, φ, so that the acute angles on each face measure 2 arctan( 1 / φ ) = arctan(2), or approximately 63.43°. A rhombus so obtained is called a golden rhombus.
Another area formula, for two sides B and C and angle θ, is = . Provided that the parallelogram is not a rhombus, the area can be expressed using sides B and C and angle at the intersection of the diagonals: [9]
The parallelogram between the pair of upright grey triangles has perpendicular diagonals in ratio , hence is a golden rhombus. If the triangle has legs of lengths 1 and 2 then each discrete spiral has length φ 2 = ∑ n = 0 ∞ φ − n . {\displaystyle \varphi ^{2}=\sum _{n=0}^{\infty }\varphi ^{-n}.}
This follows from the left side of the equation being equal to zero, requiring the right side to equal zero as well, and so the vector sum of a + b (the long diagonal of the rhombus) dotted with the vector difference a - b (the short diagonal of the rhombus) must equal zero, which indicates the diagonals are perpendicular.
This formula generalizes Heron's formula for triangles and Brahmagupta's formula for cyclic quadrilaterals. [36] Either diagonal of a rhombus divides it into two congruent isosceles triangles. Similarly, one of the two diagonals of a kite divides it into two isosceles triangles, which are not congruent except when the kite is a rhombus. [37]