Exterior angles of a triangle and the five-pointed star
Subtracting the triangle's two right angles leaves four in the exteriors
A short geometry page on the exterior angles of figures. A triangle with all three sides extended and its apex angles dotted carries the note that, throwing away the triangle's own value of two right angles, the remainder of the exterior angles is four. Beside it a five-pointed star is drawn as five triangles raised on the sides of a pentagon, justified 'by the thirteenth,' and two further views of the star show different interior segments of the pentagon removed. Faint pencil construction lines underlie the inked figures.
On this page
The exterior angles of a triangle sum to four right angles
A triangle has each side extended and its upper vertex angles marked by three dots. Throwing away the value of the triangle, which is two right angles, the remainder of the exterior angles is four.
The pentagram as five triangles on a pentagon
A five-pointed star is constructed as five triangles based on the sides of a pentagon, justified by the thirteenth. Two additional views strip out different interior segments of the pentagon to expose the star.