a, b, c

a is equal to b c, as is shown in the 2 figures marked opposite at the foot of the page.

d, e, f, n, m

d f taken from e are equal to n taken from m. n is a half portion of an eighth of a circle, which is quadruple to the circle from which d f are taken, these too being born of an eighth of a circle, one quarter of the aforesaid larger circle, that is d of one eighth and f of another.

The portion bears such proportion to the portion as the sector bears to the sector.

b c, a, d

8th, a

1/2, c, b, d

Here 1/8 of a circle is equal to 1/2 a portion of a circle, which is shown to derive from circles of quadruple proportion. And in superposing one upon the other, there remains to me, outside their common contact, the c, equal to d, or you may say b, which is equal to d. Behold, I have divided the half portion of the larger circle into 2 equal parts, that is b c. I have 2 surfaces equal in quantity and unequal in figure, and I wish to take from the one and from the other a part that are equal to one another. And first I will take, for one of those parts, the part c of the semicircle. Now I ask, how am I to find a quantity similar, or equal, in the surface a? You will never do it except by superposing one upon the other, as is shown above, [continues, written crosswise, on the left margin] and it will be c.