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

Question

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 a*b = c*d

Answer 1

This seems to be what you're after:

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