Proposition 17.
In any triangle two angles taken together in any manner are less than two right angles.
Let ABC be a triangle; I say that two angles of the triangle ABC taken together in any manner are less than two right angles. For let BC be produced to D. [Post. 2] Then, since the angle ACD is an exterior angle of the triangle ABC, it is greater than the interior and opposite angle ABC. [I. 16]
Q. E. D. 1