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On the straight line MD, and at the point M on it,
let the angle DMB be constructed equal to the angle DML, and let DB be joined.
Then, since ML is equal to MB, and MD is common,
the two sides LM, MD are equal to the two sides BM, MD respectively; and the angle LMD is equal to the angle BMD;
therefore
the base DL is equal to the base DB. [I.4]
