Chap 10 Angles

Chap 10

Angles

Some Important facts:

1.    Ray has one initial point (or end point) A and it extends in any one direction upto infinity AB and Ray BA are two different rays.

2.    Two rays, having one initial point and having opposite direction are known as opposite rays.

3.    An angle is formed by two rays with a common initial point. This common point is known as the vertex of the angle and the rays are called its arms or sides.

4.    One complete angle = 4 right angles = 360°

One straight angle = 2 right angles = 180°

One right angle = 90°

Zero angle = 0°,

0°<acute angle<90°

90°< obtuse angle<180°

180°<reflex angle<360°

5.    Two line segments or a line and a line segment or a ray and a line segment are said to be perpendicular to each other if the lines represented by them meet at right angles.

6.    Two angles in a plane are said to be adjacent angles if they have a common vertex, a common arm and the other arms on the opposite sides of the common arm.

7.    Two adjacent angles form a linear pair, if their non-common arms form a straight line.

8.    Two angles formed by two intersecting lines having no common arm are said to be vertically opposite angles. Also, vertically opposite angles are equal.

9.    Two angles whose sum is 90° form a pair of complementary angles and each angle of this pair is called the complement of the other.

10. Two angles whose sum is 180° are said to be supplementary angles and each of these two angles is called the supplementary of the other.

11. Infinitely many rays can be drawn with a given point as initial point.

12. A ray is completely known if its initial point and one more point on it are known the initial point of a ray and one more point on it are sufficient to represent it completely.

13. While naming a ray its initial point is written first.