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In geometry, **Heron's formula** (sometimes called **Hero's formula**), named after Hero of Alexandria,^{[1]} gives the area of a triangle when the lengths of all three sides are known. Unlike other triangle area formulae, there is no need to calculate angles or other distances in the triangle first.

Heron's formula states that the area of a triangle whose sides have lengths *a*, *b*, and *c* is

where *s* is the semi-perimeter of the triangle; that is,^{[2]}

Heron's formula can also be written as

Let △*ABC* be the triangle with sides *a* = 4, *b* = 13 and *c* = 15.
This triangle’s semiperimeter is

and so the area is

In this example, the side lengths and area are integers, making it a Heronian triangle. However, Heron's formula works equally well in cases where one or more of the side lengths are not integers.

The formula is credited to Heron (or Hero) of Alexandria,^{[3]} and a proof can be found in his book *Metrica*, written around AD 60. It has been suggested that Archimedes knew the formula over two centuries earlier,^{[4]} and since *Metrica* is a collection of the mathematical knowledge available in the ancient world, it is possible that the formula predates the reference given in that work.^{[5]}

A formula equivalent to Heron's, namely,

was discovered by the Chinese. It was published in *Mathematical Treatise in Nine Sections* (Qin Jiushao, 1247).^{[6]}

There are many ways to prove Heron's formula, for example using trigonometry as below, or the incenter and one excircle of the triangle,^{[7]} or as a special case of De Gua's theorem (for the particular case of acute triangles).^{[8]}

A modern proof, which uses algebra and is quite different from the one provided by Heron, follows.^{[9]}
Let *a*, *b*, *c* be the sides of the triangle and *α*, *β*, *γ* the angles opposite those sides.
Applying the law of cosines we get

From this proof, we get the algebraic statement that

The altitude of the triangle on base *a* has length *b* sin *γ*, and it follows

The difference of two squares factorization was used in two different steps.

The following proof is very similar to one given by Raifaizen.^{[10]}
By the Pythagorean theorem we have *b*^{2} = *h*^{2} + *d*^{2} and *a*^{2} = *h*^{2} + (*c* − *d*)^{2} according to the figure at the right. Subtracting these yields *a*^{2} − *b*^{2} = *c*^{2} − 2*cd*. This equation allows us to express *d* in terms of the sides of the triangle:

For the height of the triangle we have that *h*^{2} = *b*^{2} − *d*^{2}. By replacing *d* with the formula given above and applying the difference of squares identity we get

We now apply this result to the formula that calculates the area of a triangle from its height:

From the first part of the law of cotangents proof,^{[11]} we have that the triangle's area is both

and *A* = *rs*, but, since the sum of the half-angles is π/2, the triple cotangent identity applies, so the first of these is

Combining the two, we get

from which the result follows.

Heron's formula as given above is numerically unstable for triangles with a very small angle when using floating-point arithmetic. A stable alternative^{[12]}^{[13]} involves arranging the lengths of the sides so that *a* ≥ *b* ≥ *c* and computing

The brackets in the above formula are required in order to prevent numerical instability in the evaluation.

Three other area formulae have the same structure as Heron's formula but are expressed in terms of different variables. First, denoting the medians from sides *a*, *b*, and *c* respectively as *m _{a}*,

Next, denoting the altitudes from sides *a*, *b*, and *c* respectively as *h _{a}*,

Finally, denoting the semi-sum of the angles' sines as *S* = 1/2(sin *α* + sin *β* + sin *γ*), we have^{[16]}

where *D* is the diameter of the circumcircle: *D* = *a*/sin *α* = *b*/sin *β* = *c*/sin *γ*.

Heron's formula is a special case of Brahmagupta's formula for the area of a cyclic quadrilateral. Heron's formula and Brahmagupta's formula are both special cases of Bretschneider's formula for the area of a quadrilateral. Heron's formula can be obtained from Brahmagupta's formula or Bretschneider's formula by setting one of the sides of the quadrilateral to zero.

Brahmagupta's formula gives the area *K* of a cyclic quadrilateral whose sides have lengths *a*, *b*, *c*, *d* as

where *s*, the semiperimeter, is defined to be

Heron's formula is also a special case of the formula for the area of a trapezoid or trapezium based only on its sides. Heron's formula is obtained by setting the smaller parallel side to zero.

Expressing Heron's formula with a Cayley–Menger determinant in terms of the squares of the distances between the three given vertices,

illustrates its similarity to Tartaglia's formula for the volume of a three-simplex.

Another generalization of Heron's formula to pentagons and hexagons inscribed in a circle was discovered by David P. Robbins.^{[17]}

If *U*, *V*, *W*, *u*, *v*, *w* are lengths of edges of the tetrahedron (first three form a triangle; *u* opposite to *U* and so on), then^{[18]}

where

**^**"Fórmula de Herón para calcular el área de cualquier triángulo" (in Spanish). Spain: Ministerio de Educación, Cultura y Deporte. 2004. Retrieved 30 June 2012.**^**Kendig, Keith (2000). "Is a 2000-year-old formula still keeping some secrets?".*The American Mathematical Monthly*.**107**(5): 402–415. doi:10.1080/00029890.2000.12005213. JSTOR 2695295. MR 1763392. S2CID 1214184.**^**Id, Yusuf; Kennedy, E. S. (1969). "A medieval proof of Heron's formula".*The Mathematics Teacher*.**62**(7): 585–587. doi:10.5951/MT.62.7.0585. JSTOR 27958225. MR 0256819.**^**Heath, Thomas L. (1921).*A History of Greek Mathematics*. Vol. II. Oxford University Press. pp. 321–323.**^**Weisstein, Eric W. "Heron's Formula".*MathWorld*.**^**秦, 九韶 (1773). "卷三上, 三斜求积".*數學九章 (四庫全書本)*(in Chinese).**^**"Personal email communication between mathematicians John Conway and Peter Doyle". 15 December 1997. Retrieved 25 September 2020.**^**Lévy-Leblond, Jean-Marc (2020-09-14). "A Symmetric 3D Proof of Heron's Formula".*The Mathematical Intelligencer*.**43**(2): 37–39. doi:10.1007/s00283-020-09996-8. ISSN 0343-6993.**^**Niven, Ivan (1981).*Maxima and Minima Without Calculus*. The Mathematical Association of America. pp. 7–8.**^**Raifaizen, Claude H. (1971). "A Simpler Proof of Heron's Formula".*Mathematics Magazine*.**44**(1): 27–28. doi:10.1080/0025570X.1971.11976093.**^**The second part of the Law of cotangents proof depends on Heron's formula itself, but this article depends only on the first part.**^**Sterbenz, Pat H. (1974-05-01).*Floating-Point Computation*. Prentice-Hall Series in Automatic Computation (1st ed.). Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-322495-3.**^**William M. Kahan (24 March 2000). "Miscalculating Area and Angles of a Needle-like Triangle" (PDF).**^**Benyi, Arpad, "A Heron-type formula for the triangle,"*Mathematical Gazette" 87, July 2003, 324–326.***^**Mitchell, Douglas W., "A Heron-type formula for the reciprocal area of a triangle,"*Mathematical Gazette*89, November 2005, 494.**^**Mitchell, Douglas W., "A Heron-type area formula in terms of sines,"*Mathematical Gazette*93, March 2009, 108–109.**^**D. P. Robbins, "Areas of Polygons Inscribed in a Circle", Discr. Comput. Geom. 12, 223-236, 1994.**^**W. Kahan, "What has the Volume of a Tetrahedron to do with Computer Programming Languages?", [1], pp. 16–17.

- A Proof of the Pythagorean Theorem From Heron's Formula at cut-the-knot
- Interactive applet and area calculator using Heron's Formula
- J. H. Conway discussion on Heron's Formula
- "Heron's Formula and Brahmagupta's Generalization".
*MathPages.com*. - A Geometric Proof of Heron's Formula
- An alternative proof of Heron's Formula without words
- Factoring Heron