This was conjectured by Paul Erdos in 1935, and first proved by Louis Mordell in the same year. Several proofs of this inequality have been given, using Ptolemy’s theorem by Andre Avez [5], angular computations with similar triangles by Leon Bankoff [2], area inequality by V. Komornik [6], or using trigonometry by Mordell and Barrow [1]. The purpose of this note is to give another elementary proof using Ptolemy’s theorem.