Lines and Angles

A line is formed if we extend a straight path formed by joining two points in either direction endlessly. A line segment is a part of a line that is bounded by two end points. When lines or line segments meet, an angle is formed.

When the sum of the measures of two angles is 90°, the angles are called complementary angles. When the sum of the measures of two angles is 180°, the angles are called supplementary angles.

Two angles are adjacent, if they have a common vertex and a common side.

A linear pair is a pair of adjacent angles whose sum is 180o. Vertically opposite angles are the angles opposite to each other when two lines intersect. Vertically opposite angles are always equal.

If two lines intersect at a point, then the lines are intersecting lines and the common point of the lines is the point of intersection. A line that intersects two or more lines at distinct points is called a transversal.

When two lines are cut by a transversal, four interior angles, four exterior angles, four pairs of corresponding angles, two pairs of alternate interior angles, two pairs of alternate exterior angles and two pairs of interior angles on the same side of transversal are formed.

If two lines lying in the same plane do not intersect each other, then they are called parallel lines. If two parallel lines are cut by a transversal, then each pair of corresponding angles and each pair of alternate interior angles are equal in measure. Also, interior angles on the same side of the transversal are supplementary.

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