# Lines and Angles (Important Terms and Definitions)

CBSE Class 9 - Maths

1. Two angles are called adjacent angles, if

(i) they have the same vertex,

(ii) they have a common arm and

(iii) their non-common arms are on either side of the common arm.

2. Two adjacent angles are said to form a linear pair of angles, if their non-common arms are two opposite rays.

3.  If a ray stands on a line, then the sum of the adjacent angles so formed is 180°.

4. If the sum of two adjacent angles is 180°, then the non-common arms of the angles form a straight line.

5. If two lines intersect each other, then the vertically opposite angles are equal.

6. If a transversal intersects two parallel lines, then each pair of corresponding angles is equal.

7. If a transversal intersects two lines such that a pair of corresponding angles is equal, then the two lines are parallel to each other.

8. If a transversal intersects two parallel lines, then each pair of alternate interior angles is equal.

9. If a transversal intersects two lines such that a pair of alternate interior angles is equal, then the two lines are parallel.

10. If a transversal intersects two parallel lines, then each pair of interior angles on the same side of the transversal is supplementary.

11. If a transversal intersects two lines such that a pair of interior angles on the same side of the transversal is supplementary, then the two lines are parallel.

12. Lines which are parallel to the same line are parallel to each other.

13.  The sum of the three angles of a triangle is 180°.

14.  If a side of a triangle is produced, the exterior angle so formed is equal to the sum of the two interior opposite angles.