# Quick Review: Plane Geometry

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There are different terms used in geometry. They are:

###### Point
• It is a mark of position and has an exact location.
• It has no thickness, length or breadth.
###### Line
• A straight path that can be extended indefinitely in both the directions.
• It doesn’t have a fixed length.
• It has no fixed end points.
• Infinite number of points lies on the line.
###### Ray
• A straight path which can be extended indefinitely in one direction, keeping the other end fixed.
• The fixed point (end point) is known as initial point.
• It has no fixed length.
• Infinite number of points lies on a ray.
###### Line Segment
• A straight path with a definite length.
• It has two end points.
• It is a part of a line.
• Two line segments are said to be equal if they have the same length.
###### Parallel Lines
• When two lines have no point in common, then they are said to be parallel lines.
• The distance between the two parallel lines remains constant throughout.
###### Intersecting Lines
• Two lines having a common point are said to be intersecting lines.
• Two lines can intersect at most at one point only.
###### Collinear Points
• Two or more points, lying on the same plane in a line are said to be collinear points.
• The line is called "line of colinearity".
• Two points on a line are always collinear.
###### Concurrent Lines
• Three or more lines are said to be concurrent if there is a point which lies on all of them.
• The common point is called the point of concurrence.
###### Angles
There are various types of angles. They are:
• ###### Right Angle
When the angle is of 90°, it is called right angle
• ###### Acute Angle
When the angle is less than 90°, it is known as acute angle.
• ###### Obtuse Angle
When the angle is greater than 90° but less than 180°, it is called obtuse angle.
• ###### Reflex Angle
When the angle is more than 180° but less than 360°, it is called reflex angle.
• ###### Straight Angle
When the angle is of 180°, it is called straight angle.
• ###### Complete Angle
When the angle is of 360°, it is known as complete angle.
• ###### Complementary Angles
When the sum of measures of two angles is 90°, they are said to be complementary angles. Eg., 30° and 60° is a pair of complementary angles.
• ###### Supplementary Angles
When the sum of measures of two angles is 180°, they are said to be supplementary angles. Eg., 75° and 105° is a pair of supplementary angles.
Two angles are said to be adjacent if:
• They have same vertex
• They have a common arm
• Uncommon arms are on the either side of the common arm
• ###### Vertically Opposite Angles
When two lines intersect, four angles are formed. The angles opposite to each other are called vertically opposite angles.
• ###### Linear Pair
Two adjacent angles form a linear pair of angles, when their non-common arms are two opposite rays.
###### Important Points

If two parallel lines are intersected by a transversal, then

• Each pair of corresponding angles is equal.
• Each pair of alternate angles is equal.
• Interior angles on the same side of the transversal are supplementary.