Euclid I.3 (Euclid, Elements, Book I, Proposition 3)

Text: http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0086%3Abook%3D1%3Atype%3DProp%3Anumber%3D3

Given two unequal straight lines, to cut off from the greater a straight line equal to the less.

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