vertex is called angle O. This method can be used if there are no other angles at point O to cause

confusion. However, in Figure 3-2, four angles have their vertex at point O, therefore, this method

cannot be used. The letters of the sides and vertex are commonly used in geometry to identify angles.

This method is more exact and should leave no question as to which angle is referenced. In Figure 3-1,

the angle can be identified as AOB. The symbol

can be used in place of the word "angle." Thus,

angle AOB can be written as AOB or O. A symbol, such as the Greek letter , can be used to

identify an angle. This method is generally used in trigonometry.

Figure 3-3 shows the five general classes of angles. An angle that-

Is formed by perpendicular lines is a right angle.

Is less than a right angle is an acute angle.

Is greater than a right angle but less than a straight angle is called an obtuse angle.

Has sides which extend in opposite directions from the vertex is called a straight angle.

Is greater than a straight angle but less than two straight angles is called a reflex angle.