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The figure below shows an equilateral triangle ABC and a point D on the circumcircle O so that CD extended meets BA at E. If DE = d, DB = e, and DA = x, prove that x = one-half the harmonic mean of d and e, that is.

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Home | Search | Geometry | Problems | All Problems | Open Problems | Visual Index | 10 Problems | 1121-1130 | Triangle | Equilateral triangle | Circumcircle | Harmonic Mean | Similarity | Email | Solution / comment.Last updated: Jul 2, 2015 by Antonio Gutierrez.