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S		 Book IV   Proposition 7
    Therefore GK, GC, AK, FB, BK are parallelograms;
therefore
    GF is equal to HK, and GH to FK. [I.34]

And, since
    AC is equal to BD,
and
    AC is also equal to each of the straight lines GH, FK,
while
    BD is equal to each of the straight lines GF, HK, [I.34]
therefore
    the quadrilateral FGHK ie equilateral.
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