n - c a - m c - b - d f. a b and d c are equal. Take away b from both, and there remains d c, equal to a. And if you take the portion f n and set it back into the concavity f m, you have made a rectilinear triangle that stands thus (Figure), and there is left over to you a part of a portion at m. Draw the lower triangle out from the upper one.

Make the portion a equal to the sector n.

b a - d c. A way of varying the sector beneath its portion.

c - b c - d e n. When you wish to make one circle double another, you will draw the indefinite straight line d n, and then upon it you will raise the perpendicular c e, on which you will set the semidiameter a e according to the size of the circle you wish to make, and thus you will describe that circle. Then take with the compass the whole diameter of that circle c e, and set it on e d. Then draw the straight line c d, and where that straight line is tangent to the circle, or else cuts the circle, there make a point and draw the straight line e b, which line is the semidiameter of the circle that is double the aforesaid smaller circle. Again, if you make the square according to the semidiameter of the smaller circle, and make another circle of which the diameter of that square is the semidiameter, that second circle will be double the first.