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. [[I. 34](elem.1.34)]

Again, since EH is a parallelogram, and AK is its diameter, the triangle AEK is equal to the triangle AHK.