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Book II   Proposition 12
For, since the straight line CD has been cut at random at the point A,
    the square on DC
is equal to
    the squares on CA, AD and twice the rectangle contained by CA, AD. [II.4]

    Let the square on DB be added to each;
therefore
    the squares on CD, DB
are equal to
    the squares on CA, AD, DB and twice the rectangle CA, AD.
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