Two Angles of Triangle Less than Two Right Angles

Theorem

In any triangle, two angles taken together in any manner are less than two right angles.

Proof

Let $\triangle ABC$ be a triangle.

Let the side $BC$ be extended to $D$.

Since the angle $\angle ACD$ is an external angle of $\triangle ABC$, it follows that it is greater than both $\angle BAC$ and $\angle ABC$.

We add $\angle ACB$ to both, so that $\angle ACD + \angle ACB$ is greater than $\angle ABC + \angle ACB$.

But $\angle ACD + \angle ACB$ is equal to two right angles.

Therefore $\angle ABC + \angle ACB$ is less than two right angles.

In a similar manner we show that the same applies to the other two pairs of internal angles of $\triangle ABC$.

$\blacksquare$

Historical Note

This is Proposition 17 of Book I of Euclid's The Elements.