A pair of angles whose sides form two lines is called vertical angles. In the figure below, angles 1 and 3 are vertical angles since their sides form lines l and m. Similarly, angles 2 and 4 are vertical angles for the same reason.
Vertical angles are congruent and it is easy to prove. We just use the fact that a linear pair of angles are supplementary; that is their measures add up to . In the figure above, to prove that vertical angles are congruent, we have to show that and are congruent or and are congruent.
Theorem
Vertical angles are congruent.
Proof
We show that .
** Linear pair of angles are supplementary. **Linear pair of angles are supplementary. ** Substitution property of equality; that is .Substracting from both sides, we have.Therefore, vertical angles are congruent.
