Constructing a pentagon inscribed in a circle
Pentagon and pentagram in a circle, with a circle inscribed in a pentagon
Leonardo sets out how to inscribe a regular pentagon in a circle: bisect the angles a b, draw the lines a d and b c, and mark where they cut the circle at c d. The central diagram shows the pentagon with its diagonals crossing as a five-pointed star, its vertices lettered. A partial pentagon is sketched along the upper margin, and a smaller figure of a circle inscribed within a pentagon appears at the lower right.
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Inscribe a pentagon by bisecting the angles a b
To construct the pentagon, the angles a b are divided into equal parts and the lines a d and b c are drawn; where these lines cut the circle at c d gives further vertices. The accompanying figure shows the five-sided polygon with its diagonals crossing as a star, the points marked a, b, c, d.
Circle inscribed in the pentagon
A companion construction places a circle inside the pentagon, touching each of its sides. A small version of this figure is sketched at the lower right of the sheet. It is the inverse of the main problem, fitting the round figure within the straight-sided one.
Partial pentagon along the upper margin
Along the top edge of the leaf a fragment of a pentagon is sketched, only partly completed. It repeats, in abbreviated form, the five-sided figure worked out below. The cataloguer notes this partial pentagon on the upper margin.