Final answer:
To prove that 1 and 2 are supplementary angles, use the property of vertical angles to show that angle 1 and angle 2 are congruent. Then, apply the definition of supplementary angles to prove that their measures add up to 180 degrees.
Step-by-step explanation:
In order to prove that 1 and 2 are supplementary angles, we need to show that their measures add up to 180 degrees. Given that m₃ = m₄, we can use the property of vertical angles to prove that 1 and 2 are supplementary.
Vertical angles are formed by two intersecting lines and are always congruent. In this case, angle 1 and angle 3 are vertical angles, and angle 2 and angle 4 are also vertical angles. Since m₃ = m₄, we can conclude that angle 1 and angle 2 are congruent.
According to the definition of supplementary angles, if two angles are supplementary, their measures add up to 180 degrees. Since angle 1 and angle 2 are congruent, their measures are equal. Therefore, the measure of angle 1 plus the measure of angle 2 is equal to 180 degrees, proving that they are supplementary angles.