Step-by-step explanation:
Two intersecting lines form 4 angles. Each angle is equal to its opposite as vertically opposite angles are congruent.
Now, if 3 angles are equal it means 2 adjacent angles are equal too. It makes all 4 angles equal.
Since the sum of all 4 angles is 360°, each of angles is 90°.
This proves the lines are perpendicular.
