Prove that the segments formed by two internal segments to a circle crossing each other when multiplied are equal

This is a simple math question to which I had an answer a long time ago but that doesn't come to mind immediately right now.

Using the picture attached, how would I prove that ab = cd

Illustration http://img3.imageshack.us/img3/8254/circle开发者_如何学JAVAdo.jpg

---

This seems to be what you're after:

http://www.cut-the-knot.org/proofs/IntersectingChordsTheorem.shtml