Now that we have looked at Theorem 3, which stated that if a transversal makes equal alternate angles on two lines, then the lines are parallel, we can now state the converse of Theorem 3 as follows:
Converse theorem (3) If two lines are parallel, then any transversal will make equal alternate angles with them. Worked example
Given that |∠BXY| in the diagram is equal to 70 ° , fi nd each of the following angles and give the axiom or theorem used in each part to
justify your answer. ( i )
( i i )
|∠XYG| |∠XYE|
( i i i ) |∠CXY|
Solution (ii)
( i v ) |∠AXC| |∠EYF|
( v ) ( v i ) |∠GYF| A C X 70° G F Y E B
(iii) (iv)
(vi)
Practice questions 11.2
1. Copy the diagrams below of two parallel lines, and mark in two pairs of alternate angles and a transversal. Using your protractor verify that the alternate angles are equal in measure. ( i )
( ii ) F B A
2. Using the diagram below, fi nd each of the following angles and give the axiom or theorem used in each part to justify your answer.
85°
D E
C ( i ) ( i i )
|∠A| |∠B|
( i i i ) |∠C|
( i v ) |∠D| ( v )
|∠E| ( v i ) |∠F|
Section A Introducing concepts and building skills