Math bleg

by rrusczyk, Feb 13, 2009, 6:19 PM

I'm writing a section of the Precalculus book called "Surprises", which consists of surprising applications of complex numbers. That is, it's a collection of problems that don't look like complex number problems, but can be solved quickly and elegantly with complex numbers.

Here are a couple examples:

Evaluate $ \displaystyle \binom{2004}{0} - \binom{2004}{2} + \binom{2004}{4} - \cdots + \binom{2004}{2004}.$

This proof of Heron by Miles Edwards.

I'd like to have a bunch more. If you have some favorites of your own, please add them. (I'll have a whole section or two dedicated to geometry that's less surprising than the Heron proof.)

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10 Comments

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This problem:

An axb board can be tiled by 1xm tiles. Prove that either m divides a or m divides b.

It can be solved using a nice application of mth roots of unity.

by pythag011, Feb 13, 2009, 7:01 PM

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There are a ton of trig problems that would work thanks to De Moivre's theorem. Like the AIME problem last year:
\[ \arctan (1/3) + \arctan (1/4) + \arctan (1/5) = \arctan (1/n),\]
which can be solved by computing $ (3+i)(4+i)(5+i)$ and taking the real divided by the imaginary part.

by MellowMelon, Feb 13, 2009, 7:09 PM

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oh what is awesome is the coordinate of the orthocenter of three points on the unit circle (though I guess that is just normal vectors, it is very cool)

by not_trig, Feb 13, 2009, 7:12 PM

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Thanks for using my proof!

What about the classical identity
\[ \tan A  + \tan B + \tan C = \tan A \tan B \tan C , \]
where $ A$, $ B$, and $ C$ are the angles of a triangle?

by Boy Soprano II, Feb 13, 2009, 10:44 PM

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Can you give me the full citation for the proof? (The journal citation, that is.)

by rrusczyk, Feb 14, 2009, 12:41 AM

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Sure.

A Proof of Heron's Formula, The American Mathematical Monthly 114 (2007) no. 10, p. 937.

by Boy Soprano II, Feb 14, 2009, 4:37 AM

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A combinatorics problem utilizing a roots of unity filter should definitely be in the book if it is not already covered...

by archimedes1, Feb 15, 2009, 4:57 AM

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How about the proofs of the sine and cosine angle addition identities?

by CatalystOfNostalgia, Feb 19, 2009, 4:19 AM

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I always liked using complex numbers to solve geometry problems, like this one.

by SamE, Feb 21, 2009, 5:52 AM

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I'll have a couple whole sections on applications to geometry, for sure.

by rrusczyk, Feb 22, 2009, 2:53 PM

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