Euclid's Elements (Jihe Yuanben), Vol 6

...and Geng-Ji is as Wu-Ji to Ding-Ji Book 5, Proposition 11, so the two lines Ding-Ji and Geng-Ji must be equal Book 5, Proposition 9. Since the two sides Geng-Wu and Geng-Ji are respectively equal to the two sides Ding-Wu and Ding-Ji, and they share the same base Wu-Ji, angle Ding is equal to angle Geng Book 1, Proposition 8. The remaining corresponding angles, Geng-Wu-Ji and Ding-Wu-Ji, and Geng-Ji-Wu and Ding-Ji-Wu, are all equal Book 1, Proposition 4. Since angle Geng is equal to angle Jia, then angle Ding is also equal to angle Jia. Angle Ding-Wu-Ji is equal to angle Yi, and angle Ding-Ji-Wu is equal to angle Bing.

Proposition 6

If one angle of two triangles is equal, and the ratios of the two sides adjacent to the equal angles are equal, then the two figures are equiangular and the angles opposite the respective similar sides are equal.