

8.3 
Angles related to Parallel Lines and Transversals 


Parallel lines: 

Definition 
 Parallel lines will not intersect with each other.
 Parallel lines can be denoted using arrowheads as shown in the diagram below.




Transversal: 

Definition 
 A straight line that intersects two or more straight lines.
 The diagram below shows that \(PQ\) is a transversal.




The corresponding angles, alternate angles and interior angles: 

(i) Corresponding angles
 The corresponding angles are equal where \(\angle a=\angle b\).




(ii) Alternate angles
 The alternate angles are equal where \(\angle b=\angle c\).




(iii) Interior angles
 The sum of the interior angles is \(180^\circ\) where \(\angle c+\angle d=180^\circ\).




Angles of elevation and depression: 

(i) Angle of elevation 

Definition 
 When a person sees an object which is at a higher level, the angle between the line of sight and the horizontal line is known as the angle of elevation.


Form the diagram above, angle \(a\) is the angle of elevation of the bird from the sight of the boy.



(ii) Angle of Depression 

Definition 
 When a person sees an object which is at a lower level, the angle between the line of sight and the horizontal line is known as the angle of depression.


Form the diagram above, angle \(b\) is the angle of depression of the cat from the sight of the girl.


