**Lines and Angles class 9** – A line segment is a portion of a line with two endpoints where as a ray is a line with just one endpoint. 3 or more points are said to be collinear points if they lie on a same straight line. In all other conditions the points will be termed as non-collinear points. An angle is formed when 2 rays originate from a same point. There are basically 5 types of angles, namely, acute angle, obtuse angle, right angle, reflex angle, and straight angle. The angles measuring between 0 degree to 90 degree are called acute angles. If an angle is exactly equal to 90 degree then it is termed as a right angle. An obtuse angle is always greater than 90 degree and less than 180 degree. The angles measuring exactly equal to 180 degree are termed as straight angles. A reflex angle is always greater than 180 degree but less than 360 degree. If sum of two angles is 90 degree then those angles are called complementary angles, whereas in case of supplementary angles the sum of two angles is 180 degree. Two angles are adjacent, if they have a common vertex, a common arm and their non-common arms are on different sides of the common arm. According to Linear pair axiom, the sum of adjacent angles of a ray standing on a line is 180° and vice-versa. Vertically opposite angles of lines intersecting each other are equal. Lines parallel to a given line are also parallel to each other. The sum of all three angles of a triangle is always 180 degree. In a triangle, the exterior angle is always equal to the sum of the 2 interior opposite angles.

### Lines and Angles class 9 Examples