Right angle in a semicircle; angles on a line
A right triangle inscribed in a half-circle, and angles summing to two right angles
At the top are two line segments, the shorter labeled b and a. Below, two views of a half-circle standing on its diameter contain a right triangle whose right angle rests on the semicircle; the side perpendicular to the base and the portion of base outside the triangle form a rectangle at one end of the diameter. A lower row of three figures shows a perpendicular raised on a line, a segment meeting a line at a slant, and a perpendicular with four diagonals through a single point whose angles together equal two right angles. Faint text showing through from the facing folio (16r) is offset from that page, not writing on this leaf.
On this page
Right triangle inscribed in a half-circle
A right triangle is inscribed in a half-circle drawn on its diameter so that the right angle stands on the semicircle. The perpendicular side and the part of the diameter left outside the triangle form the height and base of a rectangle at the end of the diameter, with points labeled a and b.
Angles on a line equal to two right angles
Three figures show angles built on a horizontal line: first a single perpendicular, then a segment meeting the line at a diagonal, and finally a perpendicular with four further diagonals meeting at one point. The several angles gathered at that point together amount to two right angles.