If two chords intersect in the interior of a circle, then the product of the lengths of the segments of one chord is equal to the product of the lengths of the segments of the other chord.
Based on the diagram above, the relation below holds true.
This theorem is also known as the Intersecting Chords Theorem.
Consider the auxiliary segments AC and BD.
Because ∠CAB and ∠CDB are inscribed angles that intercept the same arc, CB, they are congruent. Similarly, ∠ACD≅∠ABD because they are inscribed angles that intercept the same arc, AD.
Consequently, △AEC∼△DEB because of the Angle-Angle Similarity Theorem. This allows to set the following proportions.
Finally, by cross multiplying, the desired result is obtained.