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Book VI   Proposition 17
But,
    if four straight lines be proportional, the rectangle contained by the extremes is equal to the rectangle contained by the means. [VI.16]

Therefore
    the rectangle A, C,
is equal to
    the rectangle B, D.
But
    the rectangle B, D is the square on B, for B is equal to D;
therefore
    the rectangle contained by A, C,
is equal to
    the square on B.
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