Heron formula

A formula expressing the surface area $S$ of a triangle in terms of its sides $a$, $b$ and $c$:

$$S=\sqrt{p(p-a)(p-b)(p-c)},$$

where $p=(a+b+c)/2$. Named after Heron (1st century A.D.).

References

[a1]	M. Berger, "Geometry", 1–2, Springer (1987) (Translated from French) Zbl 1153.51001
[a2]	H.S.M. Coxeter, "Introduction to geometry", Wiley (1961) Zbl 0095.34502