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Book II   Proposition 8
Again, since
    BC is equal to BD, and GK to KN,
therefore
    CK is also equal to KD, and GR to RN. [I.36]

But
    CK is equal to RN, for they are complements of the parallelogram CP; [I.43] therefore
    KD is also equal to GR;
therefore
    the four areas DK, CK, GR, RN are equal to one another.

Therefore
    the four are quadruple of CK.
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