∠a and ∠b are complementary angles. Complementary angles are two angles adjacent to each other that add up to 90°. We know this is true because of the 90° angle that is labelled.
Vertical angles are not adjacent to each other. They are across from each other, connected by the angle. They are equal in angles. It is clear that ∠a and ∠b are not equal in angles. Therefore, they are complementary angles.
