# What is the name of following formula?

What is the name of following formula? $$S=\sqrt[]{(p-a)(p-b)(p-c)(p-d)}$$ where $$a+b+c+d=2p$$ $S$ is the surface of a quadrangle.

$a, b, c, d$ are lengths of sides of a quadrangle that can be inscribed into a circle.

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@Sawarnik In Italy I've always used $p$ for semi-perimeter and $s$ for surface. –  Bakuriu Mar 10 '14 at 21:06
@Bakuriu But everywhere I have seen uses $s$ as the semi-perimeter! Do you remember Heron s formula with $p$ as the semiperimeter? –  Sawarnik Mar 10 '14 at 21:08
@Sawarnik Absolutely yes. Proof: wikipedia page on Heron's formula. –  Bakuriu Mar 10 '14 at 21:10
@Bakuriu That is really odd! But most of the world uses $s$, I think. English Wikipedia can be a proof for that. Oh, I see he is from Poland, and the Polish Wiki does use $p$! –  Sawarnik Mar 10 '14 at 21:11