How does this image prove the identity $1+2+3+4\cdots + (n-1) = \binom{n}{2}$?

Question

Possible Duplicate:
Proof for formula for sum of sequence 1+2+3+…+n?

Proof without words:

$\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad $

How does this image prove the identity $1+2+3+4\cdots + (n-1) = \binom{n}{2}$?

I found this here; could anybody explain this in a lucid manner?

Answer 1

This shows that every yellow circle uniquely determines a pair of blue circles and vice versa. The number of yellow ones is the LHS, the number of pairs of blue ones is the RHS. Cute!