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Ptolemy's theorem states that the sum of the products of the lengths of opposite sides is equal to the product of the lengths of the diagonals. When those side-lengths are expressed in terms of the sin and cos values shown in the figure above, this yields the angle sum trigonometric identity for sine: sin(α + β) = sin α cos β + cos α sin β.
Ptolemy's theorem is a relation among these lengths in a cyclic quadrilateral. = + In Euclidean geometry, Ptolemy's theorem is a relation between the four sides and two diagonals of a cyclic quadrilateral (a quadrilateral whose vertices lie on a common circle).
Given a norm, one can evaluate both sides of the parallelogram law above. A remarkable fact is that if the parallelogram law holds, then the norm must arise in the usual way from some inner product. In particular, it holds for the p {\displaystyle p} -norm if and only if p = 2 , {\displaystyle p=2,} the so-called Euclidean norm or standard norm.
The extended parallelogram sides DE and FG intersect at H. The line segment AH now "becomes" the side of the third parallelogram BCML attached to the triangle side BC, i.e., one constructs line segments BL and CM over BC, such that BL and CM are a parallel and equal in length to AH.
Another area formula, for two sides B and C and angle θ, is K = B ⋅ C ⋅ sin θ . {\displaystyle K=B\cdot C\cdot \sin \theta .\,} Provided that the parallelogram is not a rhombus, the area can be expressed using sides B and C and angle γ {\displaystyle \gamma } at the intersection of the diagonals: [ 9 ]
In a parallelogram, where both pairs of opposite sides and angles are equal, this formula reduces to = . Alternatively, we can write the area in terms of the sides and the intersection angle θ of the diagonals, as long as θ is not 90° : [ 18 ]
Parallelogram law – Sum of the squares of all 4 sides of a parallelogram equals that of the 2 diagonals; Polarization identity – Formula relating the norm and the inner product in a inner product space; Ptolemy – Astronomer and geographer (c. 100–170) Ptolemy's table of chords – 2nd century AD trigonometric table
Euler's quadrilateral theorem or Euler's law on quadrilaterals, named after Leonhard Euler (1707–1783), describes a relation between the sides of a convex quadrilateral and its diagonals. It is a generalisation of the parallelogram law which in turn can be seen as generalisation of the Pythagorean theorem .