Then, since AC is equal to CB, and CD is common,
the two sides AC, CD are equal to the two sides BC, CD;
and the angle ACD is equal to the angle BCD,
for each is right;
therefore the base AD is equal to the base DB. [I. 4]
But equal straight lines cut off equal circumferences, the greater equal to the greater, and the less to the less; [III. 28]
and each of the circumferences AD, DB is less than a semicircle;
therefore the circumference AD is equal to the circumference DB.
Complete text
Book I