In any parallelogram the complements of the parallelograms about the diameter are equal to one another.
在平行四边形中，沿对角线所成的两个补形彼此相等。
平行四边形 ABCD，对角线 AC 上取点 K；过 K 作两组平行线 EH 与 FG 分割成四个小平行四边形。两个“补形” BK 和 KD 面积相等。
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Let ABCD be a parallelogram, and AC its diameter; and about AC let EH, FG be parallelograms, and BK, KD the so-called complements; I say that the complement BK is equal to the complement KD. For, since ABCD is a parallelogram, and AC its diameter, the triangle ABC is equal to the triangle ACD.
在平行四边形内作对角线，并围出两个补形。
[I. 34] Again, since EH is a parallelogram, and AK is its diameter, the triangle AEK is equal to the triangle AHK. For the same reason the triangle KFC is also equal to KGC.
对角线两侧的大三角形相等（euclid-elements/book1-prop-034）。
Now, since the triangle AEK is equal to the triangle AHK, and KFC to KGC, the triangle AEK together with KGC is equal to the triangle AHK together with KFC. [C.N. 2] And the whole triangle ABC is also equal to the whole ADC; therefore the complement BK which remains is equal to the complement KD which remains.
这些大三角形分别由同一类小三角形和一个补形组成。
[C.N. 3] Therefore etc.
从相等整体中减去相等小三角形，余下补形相等。