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
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1@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
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@Sawarnik Absolutely yes. Proof: wikipedia page on Heron's formula. – Bakuriu Mar 10 '14 at 21:10
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2@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
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Interestingly I didn't know the name of this formula 10 mins before answering. The credit is due to another question. I get a silver badge and 115 rep for free. :D – Guy Mar 11 '14 at 15:24
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I know..the pentagon question after mine, isn't it? It was a nice Q, its inspired me to read stuff about Robbins pentagon and so...but how come a 1 line answer get 10 upvotes, I am jealous of you..btw, reputation is a useless thing. – Sawarnik Mar 11 '14 at 15:51
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@Sawarnik yep. The one after yours. It was weird though, two different people asking about pentagons in such a short while. Since when are cyclic pentagons the new trend? – Guy Mar 11 '14 at 16:05
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@Sawarnik I have no idea. Maybe because it was quick, and to the point? :D The Q&A system is weird. :p The other day I guy asking how to subtract got 7 upvotes. – Guy Mar 11 '14 at 16:07
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btw since we were talking about reputation, yeah its useless after around ~300(you need enough rep to be able to downvote, survive downvotes, and survive the rep of cost of downvoting). But if you go to the users page and sort by rep gained in the last week, I am at position 11 as of now. :D – Guy Mar 11 '14 at 16:14
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Will you come to chat, we can chat here: http://chat.stackexchange.com/rooms/13523/the-root-of-math ? – Sawarnik Mar 11 '14 at 16:16
