**Important Concepts and Formulas on Lines and Angles**

3. A line intersecting two or more given
lines in a plane at different points is called a transversal.

4. If two parallel lines are cut by atransversal, then (i) each pair of alternate angles are equal (ii) each pair of
corresponding angles are equal (iii) each pair of interior angles on the same
side of transversal are supplementary.

5. Three or more points are collinear if
all of them lie on the same line.

6. Three or more lines are concurrent if
all of them pass through the same point, and the point is called the point of
concurrence of the lines.

7. A line which cuts the given line
segment into two equal parts at 90Âº is called the perpendicular bisector.

9. Angle AOB is written as ∠AOB. Its vertex O is always written in the middle. OA and OB
are called the arms of ∠AOB or ∠BOA.

12. When two lines intersect at a point,
two pairs of vertically opposite angles are formed. Vertically opposite angles
are always equal in measure.

13. If the sum of the measures of two
angles is 90°, they form a pair of complementary angles.

14. If the sum of the measures of two
angles is 180°, they form a pair of supplementary angles.