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**Basic Terms and Definitions
Line**

A line is a collection of points along a straight path. A line extends in both the directions and has no endpoints. It has no definite length.

A line segment is a part of a line with two end points. There is only one line segment joining two given points.

A part of line with one end point is called a ray.

All the points that lie on the same line are called collinear points.

Two rays originating from the same end point form an angle.

Consider the two rays AB and AC originating from the same point A form an angle named as âˆ BAC or âˆ CAB or âˆ A. The rays that form the angle are called the arms of the angle. The end point is called the vertex of the angle. The size of an angle is measured in degrees.

**Right angle**

An angle measuring 90Â° is called a right angle. A right angle is formed by the intersection of two perpendicular lines.

Angles measuring greater than 90Â° but less than 180Â° are called obtuse angles.

An angle measuring 180Â° is called a straight angle.

Angles measuring greater than 180Â° but less than 360Â° are called reflex angles.

**Adjacent angles **

Two angles are said to be adjacent if they have a common arm and a common vertex.

**Linear pair of angles**

Two adjacent angles are said to form a linear pair if their sum is 180Â°. Non-common arms of the linear pair of angles form a straight line

**Vertically opposite angles**

When two lines intersect, four angles are formed. The angles that are opposite to each other are called vertically opposite angles. There are two pairs of vertically opposite angles.

**Complementary angles**

Two angles are said to be complementary, if their sum is 90Â°.

**Supplementary angles **

Two angles are said to be supplementary if their sum is 180Â°.

Complementary and supplementary angles may or may not be adjacent angles.

**Intersecting lines**

Two or more lines that meet at one point are called intersecting lines. When two lines intersect each other, then the vertically opposite angles are equal. Sum of all the angles formed at a point is 360Â°.

**Parallel lines**

Lines on the same plane that never intersect are called parallel lines.

**Basic Terms and Definitions
Line**

A line is a collection of points along a straight path. A line extends in both the directions and has no endpoints. It has no definite length.

A line segment is a part of a line with two end points. There is only one line segment joining two given points.

A part of line with one end point is called a ray.

All the points that lie on the same line are called collinear points.

Two rays originating from the same end point form an angle.

Consider the two rays AB and AC originating from the same point A form an angle named as âˆ BAC or âˆ CAB or âˆ A. The rays that form the angle are called the arms of the angle. The end point is called the vertex of the angle. The size of an angle is measured in degrees.

**Right angle**

An angle measuring 90Â° is called a right angle. A right angle is formed by the intersection of two perpendicular lines.

Angles measuring greater than 90Â° but less than 180Â° are called obtuse angles.

An angle measuring 180Â° is called a straight angle.

Angles measuring greater than 180Â° but less than 360Â° are called reflex angles.

**Adjacent angles **

Two angles are said to be adjacent if they have a common arm and a common vertex.

**Linear pair of angles**

Two adjacent angles are said to form a linear pair if their sum is 180Â°. Non-common arms of the linear pair of angles form a straight line

**Vertically opposite angles**

When two lines intersect, four angles are formed. The angles that are opposite to each other are called vertically opposite angles. There are two pairs of vertically opposite angles.

**Complementary angles**

Two angles are said to be complementary, if their sum is 90Â°.

**Supplementary angles **

Two angles are said to be supplementary if their sum is 180Â°.

Complementary and supplementary angles may or may not be adjacent angles.

**Intersecting lines**

Two or more lines that meet at one point are called intersecting lines. When two lines intersect each other, then the vertically opposite angles are equal. Sum of all the angles formed at a point is 360Â°.

**Parallel lines**

Lines on the same plane that never intersect are called parallel lines.