Answer:
Two angles whose sum is 90° (that is, one right angle) are called complementary angles and one is called the complement of the other. Here, ∠AOB and ∠BOC are called complementary angles. ∠AOB is complement of ∠BOC and ∠BOC is complement of ∠AOB. Thus, the complementary angle of 60° is the angle measure 30°.
