Vertical angles are formed by two intersecting lines and are congruent. In this case, angle 2 has vertical angles with angles 1 and 3. Therefore, angle 3 is a vertical angle with angle 2.
Two intersecting lines create four angles, numbered 1, 2, 3, and 4, where ∠4 is adjacent to ∠1 and ∠3, and m∠2=110°. Which other angle has a measure that is also equal to 110°?
Since angle 4 is adjacent to angles 1 and 3, it is also adjacent to angle 2. Adjacent angles share a common vertex and a common side. Therefore, angle 4 is adjacent to angle 2, and if the measure of angle 2 is 110°, then the measure of angle 4 is also 110°.
