Quick Review: Plane Geometry

In this article, you will learn the basic terms and concepts related to 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.

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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.

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Adjacent 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.

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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.

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