In equal circles equal straight lines cut off equal circumferences, the greater equal to the greater and the less to the less.
IN æqualibus circulis, æquales rectæ lineæ æquales peripherias auferunt, maiorem quidem maiori, minorem autem minori.
等圜內之直線等。則其割本圜之分。大與大。小與小。各等。
Let ABC, DEF be equal circles, and in the circles let AB, DE be equal straight lines cutting off ACB, DFE as greater circumferences and AGB, DHE as lesser; I say that the greater circumference ACB is equal to the greater circumference DFE, and the less circumference AGB to DHE.
For let the centres K, L of the circles be taken, and let AK, KB, DL, LE be joined.
Now, since the circles are equal,
the radii are also equal;
therefore the two sides AK, KB are equal to the two sides DL, LE;
and the base AB is equal to the base DE;
therefore the angle AKB is equal to the angle DLE. [I. 8]
But equal angles stand on equal circumferences, when they are at the centres; [III. 26]
therefore the circumference AGB is equal to DHE.
And the whole circle ABC is also equal to the whole circle DEF;
therefore the circumference ACB which remains is also equal to the circumference DFE which remains.
Complete text
Book I