Note 16 — BOOK III. (continued 2)

This too may be easily effected by means of the first problem, and the present. Thus let a b c be a right angle, which it is required to trisect; then, upon the side a b, describe an equilateral triangle c d b, and bisect the angle d b c, and the angles a b d, d b e, e b c, shall be equal. For the angle d b c, is one third of two right angles, or two thirds of one right, and consequently the angle d b a, is one third of a right angle; and this is equal to d b e, the half of d b c. Therefore they are all equal.