- An angle is formed when two lines or rays or line segments meet or intersect.
- When the sum of the measures of two angles is 90°, the angles are called complementary angles.
- Each of them is called complement of the other.
- When the sum of the measures of two angles is 180°, the angles are called supplementary angles.
- Each of them is called supplement of the other.
- Two angles are called adjacent angles, if they have a common vertex and a common arm but no common interior points.
- A linear pair is a pair of adjacent angles whose non-common sides are opposite rays.
- When two lines intersect, the vertically opposite angles so formed are equal.
- When two lines are intersected by a transversal, eight angles are formed.
- These angles can be classified as 4 interior angles, 4 exterior angles, 4 pairs of corresponding angles, 2 pairs of alternate interior angles, 2 pairs of alternate exterior angles and two pairs of interior angles on the same side of the transversal.
- If two parallel lines are intersected by a transversal,
(i) each pair of corresponding angles is equal.
(ii) each pair of alternate interior angles is equal.
(iii) each pair of interior angles on the same side of the transversal is
supplementary. - Converses of the above results are also true.
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