Cyclic Quadrilateral Property Problem

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In a circle with center O, chord AB equals chord AC. Chord AD cuts BC in E. If AC = 12 and AE = 8, then what is the length of AD.

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  1. Draw the perpendicular bisector of BC. It will go through O and A. Let it cut BC at X.

  2. Reflect AED about AOX to get AE’D’ where E’ and D’ will be on BC and on the circle respectively.

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  1. Use Pythagoras theorem on AXC and on AXE to get $12^2 – 8^2 = XC^2 – XE^2$

  2. The RHS $= (XC + XE)[XC – XE] = (CE’)[BE’]$

  3. By power of a point, $ (CE’)[E’B] = AE’ \times E'D’ = 8 \times ED’ = 8 \times ED$. Result follows