Parallel Lines

EscalatorBig.jpgLines, on the same plane, are parallel if they never meet and always remain the same distance apart.

Parallel lines are shown by a small arrow on each.

A line crossing parallel lines is called a transversal.

 Y9_Parallel_Lines_01.gif

Angles and Parallel Lines

Because of the symmetry of parallel lines, there are three important angle properties involving parallel lines and a transversal.

Types of angles
Diagram
Property
Abbreviation
Corresponding angles
Y9_Parallel_Lines_02.gif

Corresponding angles are equal.

e.g. a = b

(corr. ∠'s on Y9_Parallel_Lines_03.giflines)

Example
Alternate angles
Y9_Parallel_Lines_04.gif

Alternate angles are equal.

e.g. c = d

(alt. ∠'s on Y9_Parallel_Lines_03.gif lines)

Example
Co-interior angles
Y9_Parallel_Lines_05.gif

Co-interior angles are supplementary.

e.g. d + e = 180°

(coint. ∠'s on Y9_Parallel_Lines_03.gif lines)

Example

Each diagram has several pairs of corresponding, alternate and co-interior angles.