Angle formed by rays OA and OB
Definition: An angle is formed when two rays meet at a common endpoint.
The two rays are called the arms of an angle and the point that unites them is called the vertex.
In the adjacent figure(top), the two rays OA and OB having a common endpoint ‘O’ form an angle AOB.
OA and OB are called the arms of angle AOB and O is called the vertex of angle AOB.
Naming angles: An angle can be named in several ways.
i. One way is to use the ∠ symbol accompanied by three letters. The first and third letters indicate points on the two rays. The letter in the middle is the vertex.
Ex: ∠AOB as shown in figure at the bottom.
Note that the first and third letters are interchangeable because they both measure the same angle i.e, ∠AOB = ∠BOA
ii. Another way to label an angle is by just using the ∠ symbol accompanied by the vertex point alone i.e, ∠O.
However, this method only works when there is only one angle at the vertex point. If more than one angle is formed at a vertex point, we need to specify which angle we are talking about by naming it in a different way.
iii. A third way to label an angle is by using the ∠ symbol accompanied by the letter or number shown between the angle i.e, ∠θ or ∠30°
θ is the most common symbol to represent an angle. The symbols α β γ etc are also used.