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the circumference BC is equal to CD,
the angle BFC is also equal to the angle CFD. [III.27]
And
the angle BFC is double of the angle KFC, and the angle DFC of the angle LFC;
therefore
the angle KFC is also equal to the angle LFC.
But
the angle FCK is also equal to the angle FCL;
therefore
FKC, FLC are two triangles having two angles equal to two angles and one side equal to one side, namely FC which is common to them;
therefore they will also have the remaining sides equal to the remaining sides, and the remaining angle to the remaining angle. [I.26]
