Question

If two angles have a common vertex and their arms form opposite rays (Figure), Then, how many angles are formed?

Answer 1

A total of 13 angles are formed, namely ∠AOB, ∠BOC, ∠COD, ∠DOA, ∠AOC, ∠BOD, ∠DOB, ∠AOD, ∠BOA, ∠COB, ∠DOC, ∠AOA.