Formula for the area of a quadrilateral
A quadrilateral. In geometry , Bretschneider's formula is a mathematical expression for the area of a general quadrilateral . It works on both convex and concave quadrilaterals (but not crossed ones), whether it is cyclic or not.
History [ edit ] The German mathematician Carl Anton Bretschneider discovered the formula in 1842. The formula was also derived in the same year by the German mathematician Karl Georg Christian von Staudt .
Formulation [ edit ] Bretschneider's formula is expressed as:
K = ( s − a ) ( s − b ) ( s − c ) ( s − d ) − a b c d ⋅ cos 2 ( α + γ 2 ) {\displaystyle K={\sqrt {(s-a)(s-b)(s-c)(s-d)-abcd\cdot \cos ^{2}\left({\frac {\alpha +\gamma }{2}}\right)}}} = ( s − a ) ( s − b ) ( s − c ) ( s − d ) − 1 2 a b c d [ 1 + cos ( α + γ ) ] . {\displaystyle ={\sqrt {(s-a)(s-b)(s-c)(s-d)-{\tfrac {1}{2}}abcd[1+\cos(\alpha +\gamma )]}}.} Here, a , b , c , d are the sides of the quadrilateral, s is the semiperimeter , and α and γ are any two opposite angles, since cos ( α + γ ) = cos ( β + δ ) {\displaystyle \cos(\alpha +\gamma )=\cos(\beta +\delta )} as long as α + β + γ + δ = 360 ∘ . {\displaystyle \alpha +\beta +\gamma +\delta =360^{\circ }.}
Denote the area of the quadrilateral by K . Then we have
K = a d sin α 2 + b c sin γ 2 . {\displaystyle {\begin{aligned}K&={\frac {ad\sin \alpha }{2}}+{\frac {bc\sin \gamma }{2}}.\end{aligned}}} Therefore
2 K = ( a d ) sin α + ( b c ) sin γ . {\displaystyle 2K=(ad)\sin \alpha +(bc)\sin \gamma .} 4 K 2 = ( a d ) 2 sin 2 α + ( b c ) 2 sin 2 γ + 2 a b c d sin α sin γ . {\displaystyle 4K^{2}=(ad)^{2}\sin ^{2}\alpha +(bc)^{2}\sin ^{2}\gamma +2abcd\sin \alpha \sin \gamma .} The law of cosines implies that
a 2 + d 2 − 2 a d cos α = b 2 + c 2 − 2 b c cos γ , {\displaystyle a^{2}+d^{2}-2ad\cos \alpha =b^{2}+c^{2}-2bc\cos \gamma ,} because both sides equal the square of the length of the diagonal BD . This can be rewritten as
( a 2 + d 2 − b 2 − c 2 ) 2 4 = ( a d ) 2 cos 2 α + ( b c ) 2 cos 2 γ − 2 a b c d cos α cos γ . {\displaystyle {\frac {(a^{2}+d^{2}-b^{2}-c^{2})^{2}}{4}}=(ad)^{2}\cos ^{2}\alpha +(bc)^{2}\cos ^{2}\gamma -2abcd\cos \alpha \cos \gamma .} Adding this to the above formula for 4K 2 yields
4 K 2 + ( a 2 + d 2 − b 2 − c 2 ) 2 4 = ( a d ) 2 + ( b c ) 2 − 2 a b c d cos ( α + γ ) = ( a d + b c ) 2 − 2 a b c d − 2 a b c d cos ( α + γ ) = ( a d + b c ) 2 − 2 a b c d ( cos ( α + γ ) + 1 ) = ( a d + b c ) 2 − 4 a b c d ( cos ( α + γ ) + 1 2 ) = ( a d + b c ) 2 − 4 a b c d cos 2 ( α + γ 2 ) . {\displaystyle {\begin{aligned}4K^{2}+{\frac {(a^{2}+d^{2}-b^{2}-c^{2})^{2}}{4}}&=(ad)^{2}+(bc)^{2}-2abcd\cos(\alpha +\gamma )\\&=(ad+bc)^{2}-2abcd-2abcd\cos(\alpha +\gamma )\\&=(ad+bc)^{2}-2abcd(\cos(\alpha +\gamma )+1)\\&=(ad+bc)^{2}-4abcd\left({\frac {\cos(\alpha +\gamma )+1}{2}}\right)\\&=(ad+bc)^{2}-4abcd\cos ^{2}\left({\frac {\alpha +\gamma }{2}}\right).\end{aligned}}} Note that: cos 2 α + γ 2 = 1 + cos ( α + γ ) 2 {\displaystyle \cos ^{2}{\frac {\alpha +\gamma }{2}}={\frac {1+\cos(\alpha +\gamma )}{2}}} (a trigonometric identity true for all α + γ 2 {\displaystyle {\frac {\alpha +\gamma }{2}}} )
Following the same steps as in Brahmagupta's formula , this can be written as
16 K 2 = ( a + b + c − d ) ( a + b − c + d ) ( a − b + c + d ) ( − a + b + c + d ) − 16 a b c d cos 2 ( α + γ 2 ) . {\displaystyle 16K^{2}=(a+b+c-d)(a+b-c+d)(a-b+c+d)(-a+b+c+d)-16abcd\cos ^{2}\left({\frac {\alpha +\gamma }{2}}\right).} Introducing the semiperimeter
s = a + b + c + d 2 , {\displaystyle s={\frac {a+b+c+d}{2}},} the above becomes
16 K 2 = 16 ( s − d ) ( s − c ) ( s − b ) ( s − a ) − 16 a b c d cos 2 ( α + γ 2 ) {\displaystyle 16K^{2}=16(s-d)(s-c)(s-b)(s-a)-16abcd\cos ^{2}\left({\frac {\alpha +\gamma }{2}}\right)} K 2 = ( s − a ) ( s − b ) ( s − c ) ( s − d ) − a b c d cos 2 ( α + γ 2 ) {\displaystyle K^{2}=(s-a)(s-b)(s-c)(s-d)-abcd\cos ^{2}\left({\frac {\alpha +\gamma }{2}}\right)} and Bretschneider's formula follows after taking the square root of both sides:
K = ( s − a ) ( s − b ) ( s − c ) ( s − d ) − a b c d ⋅ cos 2 ( α + γ 2 ) {\displaystyle K={\sqrt {(s-a)(s-b)(s-c)(s-d)-abcd\cdot \cos ^{2}\left({\frac {\alpha +\gamma }{2}}\right)}}} The second form is given by using the cosine half-angle identity
cos 2 ( α + γ 2 ) = 1 + cos ( α + γ ) 2 , {\displaystyle \cos ^{2}\left({\frac {\alpha +\gamma }{2}}\right)={\frac {1+\cos \left(\alpha +\gamma \right)}{2}},} yielding
K = ( s − a ) ( s − b ) ( s − c ) ( s − d ) − 1 2 a b c d [ 1 + cos ( α + γ ) ] . {\displaystyle K={\sqrt {(s-a)(s-b)(s-c)(s-d)-{\tfrac {1}{2}}abcd[1+\cos(\alpha +\gamma )]}}.} Emmanuel García has used the generalized half angle formulas to give an alternative proof. [1]
Related formulae [ edit ] Bretschneider's formula generalizes Brahmagupta's formula for the area of a cyclic quadrilateral , which in turn generalizes Heron's formula for the area of a triangle .
The trigonometric adjustment in Bretschneider's formula for non-cyclicality of the quadrilateral can be rewritten non-trigonometrically in terms of the sides and the diagonals e and f to give[2] [3]
K = 1 4 4 e 2 f 2 − ( b 2 + d 2 − a 2 − c 2 ) 2 = ( s − a ) ( s − b ) ( s − c ) ( s − d ) − 1 4 ( ( a c + b d ) 2 − e 2 f 2 ) . {\displaystyle {\begin{aligned}K&={\tfrac {1}{4}}{\sqrt {4e^{2}f^{2}-(b^{2}+d^{2}-a^{2}-c^{2})^{2}}}\\&={\sqrt {(s-a)(s-b)(s-c)(s-d)-{\tfrac {1}{4}}((ac+bd)^{2}-e^{2}f^{2})}}.\end{aligned}}} References & further reading [ edit ] Ayoub, Ayoub B. (2007). "Generalizations of Ptolemy and Brahmagupta Theorems". Mathematics and Computer Education . 41 (1). ISSN 0730-8639 . C. A. Bretschneider. Untersuchung der trigonometrischen Relationen des geradlinigen Viereckes. Archiv der Mathematik und Physik, Band 2, 1842, S. 225-261 (online copy, German ) F. Strehlke: Zwei neue Sätze vom ebenen und sphärischen Viereck und Umkehrung des Ptolemaischen Lehrsatzes . Archiv der Mathematik und Physik, Band 2, 1842, S. 323-326 (online copy, German ) External links [ edit ]