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the angle FCK is right,
therefore
the square on FK is equal to the squares on FC, CK. [I.47]
For the same reason
the square on FK is also equal to the squares on FB, BK;
so that
the squares on FC, CK are equal to the squares on FB, BK, of which the square on FC is equal to the square on FB;
therefore
The square on CK which remains is equal to the square on BK.
