The theorem of the gnomon states that certain parallelograms occurring in a gnomon have areas of equal size.
Theorem
In a parallelogram ABCD with a point P on the diagonal A C {\displaystyle AC} AC the parallel to AD through P intersects the side CD in G G and the side AB in H. Similarly the parallel to the side A AB through P intersects the side AD in I and the side BC in F. The theorem of the gnomon now states, that the parallelograms \( {\displaystyle HBFP} \) and \( {\displaystyle IPGD} \) have equal areas.[1][2]
Gnomon is the name for the L-shaped figure consisting of the two overlapping parallelograms \( {\displaystyle ABFI} \) and A H G D {\displaystyle AHGD} {\displaystyle AHGD}. The parallelograms of equal area \( {\displaystyle HBFP} \) and \( {\displaystyle IPGD} \) are called complements (of the parallelograms on diagonal \( {\displaystyle PFCG} \) and \( {\displaystyle AHPI}) \) .[3]
Gnomon: \( {\displaystyle ABFPGD} \)
Theorem of the Gnomon: green area = red area,
\( {\displaystyle |AHGD|=|ABFI|,\,|HBFP|=|IPGD|} \)
Proof
The proof of the theorem is straightforward if one considers the areas of the main parallelogram and the two inner parallelograms around its diagonal:
first, the difference between the main parallelogram and the two inner parallelograms is exactly equal to the combined area of the two complements;
second, all three of them are bisected by the diagonal. This yields:[4]
\( {\displaystyle |IPGD|={\frac {|ABCD|}{2}}-{\frac {|AHPI|}{2}}-{\frac {|PFCG|}{2}}=|HBFP|} \)
Applications and extensions
geometrical representation of a division
Transferring the ratio of a partition of line segment AB to line segment HG: \( {\displaystyle {\tfrac {|AH|}{|HB|}}={\tfrac {|HP|}{|PG|}}} \)
The theorem of the gnomon can be used to construct a new parallelogram or rectangle of equal area to a given parallelogram or rectangle by the means of straightedge and compass constructions. This also allows the representation of a division of two numbers in geometrical terms, an important feature to reformulate geometrical problems in algebraic terms. More precisely, if two numbers are given as lengths of line segments one can construct a third line segment, the length of which matches the quotient of those two numbers (see diagram). Another application is to transfer the ratio of partition of one line segment to another line segment (of different length), thus dividing that other line segment in the same ratio as a given line segment and its partition (see diagram).[1]
\( \mathbb {A} \) is the (lower) parallepiped around the diagonal with P and its complements \( \mathbb {B} \), \( \mathbb {C}\) and \( \mathbb {D} \) have the same volume: \( {\displaystyle |\mathbb {B} |=|\mathbb {C} |=|\mathbb {D} |}\)
A similar statement can be made in three dimensions for parallelepipeds. In this case you have a point P on the space diagonal of a parallelepiped, and instead of two parallel lines you have three planes through P, each parallel to the faces of the parallelepiped. The three planes partition the parallelepiped into eight smaller parallelepipeds; two of those surround the diagonal and meet at P. Now each of those two parallepipeds around the diagonal has three of the remaining six parallelepipeds attached to it, and those three play the role of the complements and are of equal volume (see diagram).[2]
General theorem about nested parallelograms
general theorem:
grenn area = blue area - red area
The theorem of gnomon is special case of a more general statement about nested parallelograms with a common diagonal. For a given parallelogram ABCD consider an arbitrary inner parallelogram \( {\displaystyle AFCE} \) having AC as a diagonal as well. Furthermore there are two uniquely determined parallelograms \( {\displaystyle GFHD} \) and \( {\displaystyle IBJF} \) the sides of which are parallel to the sides of the outer parallelogram and which share the vertex F with the inner parallelogram. Now the difference of the areas of those two parallelograms is equal to area of the inner parallelogram, that is:[2]
\( {\displaystyle |AFCE|=|GFHD|-|IBJF|} \)
This statement yields the theorem of the gnomon if one looks at a degenerate inner parallelogram \( {\displaystyle AFCE} \) whose vertices are all on the diagonal AC. This means in particular for the parallelograms \( {\displaystyle GFHD} \) and \( {\displaystyle IBJF} \), that their common point F is on the diagonal and that the difference of their areas is zero, which is exactly what the theorem of the gnomon states.
Historical aspects
The theorem of the gnomon was described as early as in Euclid's Elements (around 300 BC) and there it plays an important role in the derivation of other theorems. It is given as proposition 43 in Book I of the Elements, where it is phrased as a statement about parallelograms without using the term gnomon. The latter is introduced by Euclid as the second definition of the second book of Elements. Further theorems for which the gnomon and its properties play an important role are proposition 6 in Book II, proposition 29 in Book VI and propositions 1 to 4 in Book XIII.[5][4][6]
References
Lorenz Halbeisen, Norbert Hungerbühler, Juan Läuchli: Mit harmonischen Verhältnissen zu Kegelschnitten: Perlen der klassischen Geometrie. Springer 2016, ISBN 9783662530344, pp. 190–191 (German)
George W. Evans: Some of Euclid's Algebra. The Mathematics Teacher, Vol. 20, No. 3 (March 1927), pp. 127–141 (JSTOR)
William J. Hazard: Generalizations of the Theorem of Pythagoras and Euclid's Theorem of the Gnomon. The American Mathematical Monthly, Vol. 36, No. 1 (January 1929), pp. 32–34 (JSTOR)
Paolo Vighi, Igino Aschieri: From Art to Mathematics in the Paintings of Theo van Doesburg. In: Vittorio Capecchi, Massimo Buscema, Pierluigi Contucci, Bruno D'Amore (editors): Applications of Mathematics in Models, Artificial Neural Networks and Arts. Springer, 2010, ISBN 9789048185818, pp. 601–610
External links
Wikimedia Commons has media related to Gnomons (geometry).
Theorem of the gnomon and Definition of the gnomon in Euclid's Elements
Notes
Lorenz Halbeisen, Norbert Hungerbühler, Juan Läuchli: Mit harmonischen Verhältnissen zu Kegelschnitten: Perlen der klassischen Geometrie. Springer 2016, ISBN 9783662530344, pp. 190-191
William J. Hazard: Generalizations of the Theorem of Pythagoras and Euclid's Theorem of the Gnomon. The American Mathematical Monthly, volume 36, no. 1 (Jan., 1929), pp. 32–34 (JSTOR)
Johannes Tropfke: Geschichte der Elementarmathematik Ebene Geometrie – Band 4: Ebene Geometrie. Walter de Gruyter, 2011, ISBN 9783111626932, pp. 134-135 (German)
Roger Herz-Fischler: A Mathematical History of the Golden Number. Dover, 2013, ISBN 9780486152325, pp.35–36
Paolo Vighi, Igino Aschieri: From Art to Mathematics in the Paintings of Theo van Doesburg. In: Vittorio Capecchi, Massimo Buscema, Pierluigi Contucci, Bruno D'Amore (editors): Applications of Mathematics in Models, Artificial Neural Networks and Arts. Springer, 2010, ISBN 9789048185818, pp. 601–610, in particular pp. 603–606
George W. Evans: Some of Euclid's Algebra. The Mathematics Teacher, Volume 20, no. 3 (March, 1927), pp. 127–141 (JSTOR)
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