Proofs Without Words

This is a great collection of favorite Proofs Without Words from the community at MathOverflow.net:

http://mathoverflow.net/questions/8846/proofs-without-words

MathOverflow is geared toward research-level mathematics, so many of the examples deal with very advanced ideas.  However, there are some lovely and accessible proofs of the sum of the first n squares, the sum of the first n cubes,  the AM-GM inequality, and various Fibonacci identities.

And since it’s MathOverflow, the comments include a healthy debate about whether you can actually prove something with only a diagram!