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Book II   Proposition 4
    CGKB was shown to be equilateral and and also right-angled; therefore
    CK is a square;
and
    it is described on CB.

For the same reason
    HF is also a square;
and
    it is described on HG, that is AC. [I.34]

Therefore
    the squares HF, CK are the squares on AC, CB.
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