Now, since the circumference BGC is equal to the circumference EHF,
the angle BKC is also equal to the angle ELF. [III. 27]
And, since the circles ABC, DEF are equal, the radii are also equal;
therefore the two sides BK, KC are equal to the two sides EL, LF;
and they contain equal angles;
therefore the base BC is equal to the base EF. [I. 4]