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Lines PQ and RS are intersecting lines because they are intersecting each other at
point ‘O’.

Non-intersecting Lines (or Parallel lines)

The lines which never intersect each other at any point are called Non-
intersecting lines. These lines are called Parallel Lines and the common length
between two lines is the distance between parallel lines.

Lines AB and CD are non-intersecting (parallel) lines.

9. Pairs of Angles
Linear Pair of Angles: When the sum of two adjacent angles is 180°, then they
are called a linear pair of angles.

Linear pair Axiom
(i) If a ray stands on a line, then the sum of two adjacent angles so formed is 180°.
(ii) If the sum of two adjacent angles is 180°, then a ray stands on a line (i.e. the
non-common arms form a line).
AOC + BOC = 180°