Euclid, Elements of Geometry, Book I, Proposition 16 (Edited by Dionysius Lardner, 1855)

Proposition XVI. Theorem.
[Euclid, ed. Lardner, 1855, on Google Books]

 (89) If one side (B C) of a triangle (B A C) be produced, the external angle (A C D) is greater than either of the internal opposite angles (A or B.)

For bisect the side A C in E (X), draw B E and produce it until E F be equal to B E (III), and join F C.

The triangles C E F and A E B have the sides C E and E F equal to the sides A E and E B (const.), and the angle C E F equal to A E B (XV), therefore the angles E C F and A are equal (IV), and therefore A C D is greater than A. In like manner it can be shown, that if A C be produced, the external angle B C G is greater than the angle B, and therefore that the angle A C D, which is equal to B C G (XV), is greater than the angle B.

(90)   Cor. 1.—Hence it follows, that each angle of a triangle is less than the supplement of either of the other angles (84). For the external angle is the supplement of the adjacent internal angle (XIII).

(91)   Cor. 2.—If one angle of a triangle be right or obtuse, the others must be acute. For the supplement of a right or obtuse angle is right or acute (82), and each of the other angles must be less than this supplement, and must therefore be acute.

(92)   Cor. 3.—More than one perpendicular cannot be drawn from the same point to the same right line. For if two lines be supposed to be drawn, one of which is perpendicular, they will form a triangle having one right angle. The other angles must therefore be acute (91), and therefore the other line is not perpendicular.

(93)   Cor. 4.—If from any point a right line be drawn to a given right line, making with it an acute and obtuse angle, and from the same point a perpendicular be drawn, the perpendicular must fall at the side of the acute angle. For otherwise a triangle would be formed having a right and an obtuse angle, which cannot be (91).

(94)   Cor. 5.—The equal angles of an isosceles triangle must be both acute.

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