For let DU, UE, BS, SG be joined.
Then, since DO is parallel to PE, the alternate angles DOU, UPE are equal to one another. [I. 29]
And, since DO is equal to PE, and OU to UP, and they contain equal angles,
therefore the base DU is equal to the base UE, the triangle DOU is equal to the triangle PUE, and the remaining angles are equal to the remaining angles; [I. 4]
therefore the angle OUD is equal to the angle PUE.
For this reason DUE is a straight line. [I. 14]
For the same reason, BSG is also a straight line, and BS is equal to SG.
Now, since CA is equal and parallel to DB, while CA is also equal and parallel to EG,
therefore DB is also equal and parallel to EG. [XI. 9]
And the straight lines DE, BG join their extremities;
therefore DE is parallel to BG. [I. 33]
Therefore the angle EDT is equal to the angle BGT, for they are alternate; [I. 29]
and the angle DTU is equal to the angle GTS. [I. 15]
Therefore DTU, GTS are two triangles which have two angles equal to two angles, and one side equal to one side, namely that subtending one of the equal angles, that is, DU equal to GS,
for they are the halves of DE, BG;
therefore they will also have the remaining sides equal to the remaining sides. [I. 26]
Therefore DT is equal to TG, and UT to TS.
Complete text
Book I