Euclid's Elements (Jihe Yuanben), Vol 6

The ratio of the base of 1 (Yi-Bing) to the base of 2 (Wu-Ji) is the same as the ratio of 3 (triangle Jia-Yi-Bing) to 4 (triangle Ding-Wu-Ji) Volume 5, 6. Furthermore, the two parallelograms Geng-Wu and Wu-Xin are each twice the size of the two triangles Jia-Yi-Bing and Ding-Wu-Ji Volume 1, 33. Since the ratio of Jia-Yi-Bing to Ding-Wu-Ji is equal to the ratio of their bases Yi-Bing and Wu-Ji, the ratio of the two parallelograms Geng-Wu and Wu-Xin is also equal to the ratio of the two bases Yi-Bing and Wu-Ji Volume 5, 15. Alternatively, from Ren, Gui, Zi, and Chou, Yin, draw straight lines parallel to Geng-Yi and Xin-Ji; this will make the proof evident based on the reasoning above.

Additional Proposition: For any two triangles and two parallelograms, if they have equal bases, the ratio between them is equal to the ratio of the heights of the two figures.