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Is there a site to search for formulas? I often come up with formulas that I would like to know the name, where they are used or what they are related to. One would type, for example, $\sin(a)\cos(b)+\sin(b)\cos(a)$ and it would intelligently associate it with the sum of sines formula. I know this would be no trivial service but I find it would be a great tool.

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I think there was no need for me to check but anyway I just checked. Wolfram Alpha did it. – user21436 Mar 15 '12 at 9:41
There are many lists of formulae on the internet, but if you want something which will parse what you write then you want a computer algebra system – Henry Mar 15 '12 at 9:46

Unfortunately the existing search engines do not have that capability yet. You may check back in a year, there will be. (Trust me!)

In the meanwhile, maybe this one can be of some help

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What about this?

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+1. See my comment. – user21436 Mar 15 '12 at 9:42
or even this link (see "alternate form") – Henry Mar 15 '12 at 9:44
ac/(b^2-a^2) won't say it's to the closest point of a line in relation to origin - which is something I'd like to check ): – Dokkat Mar 15 '12 at 15:21
Actually, the best would probably be "What about Wolfram Alpha?" with a link attached to the words Wolfram Alpha. – Graphth Mar 16 '12 at 17:40

I think more specifically Dokkat is looking for (something of this sort)

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Wolfram|Alpha is the kind of thing you're looking for. For now, unfortunately, there aren't so many engines which allows you to search for knowledge rather than just results. For the sake of posting an answer though, I have heard about The Springer LaTeX search but I'm not sure how useful could it be.

Morals of the story, you can always use a combination of Google, W|A, math.SE search bar, and question posting.

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Thanks for leaving a message on my blog. You may read more there. I am anyway back and I hope I won't regret it. – user21436 Mar 16 '12 at 16:26
@KannappanSampath I'm glad you're back! – user2468 Mar 16 '12 at 16:27

The search engine at lets you search for formulas by typing in keywords (it also provides search suggestions). It searches Wikipedia's vast array of formulas as well as user-shared formulas and a small but growing formula database of their own. It does not however provide a means to search based on the source such as sin(a)cos(b)+sin(b)cos(a) as you requested, but it does do the reverse, for example|ev|Sine%20sum%20and%20difference%20identity.

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