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S		 Book II   Proposition 13
For, since the straight line CB has been cut at random at the point D,
    the squares on CB and BD
are equal to
    twice the rectangle contained by CB, BD and the square on DC. [II.7]

    Let the square on DA be added to each;
therefore
    the squares on CB, BD, DA
are equal to
    twice the rectangle contained by CB, BD and the squares on AD, DC.
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