GEOMETRIC TERMS AND DEFINITIONS

Points :

We use a point to mark a location or position.

Examples of points are :

(i)  the corner of your desk

(ii)  the tip of your compass needle.

The letters A, B and C identify the points.

Vertices :

The figure alongside contains four points which have been labelled A, B, C and D. These corner points are also known as vertices.

Straight Line :

A straight line, usually just called a line, is a continuous infinite collection of points with no beginning or end which lie in a particular direction.

Line segment :

[AB] is the line segment which joins points A and B. We call it “line segment AB” or “line segment BA”. It is only part of the line (AB).

Ray :

[AB) is the ray which starts at A, passes through B, then continues on forever in that direction.

Collinear Points :

If three or more points lie on a single straight line, we say that the points are collinear.

Concurrent lines :

If three or more lines meet or intersect at the same point, we say that the lines are concurrent. The lines shown are concurrent at point B.

Parallel lines :

Parallel lines are lines which are always a fixed distance apart and never meet.

Angle :

An angle is made up of two arms which meet at a point called the vertex.

Revolution :

One complete turn. 

One revolution  =  360°

Straight angle :

1/2 turn

1 straight angle  =  180°

Right angle :

1/4 turn

1 right angle  =  90°

Acute angle :

Less than a 1/4 turn.

An acute angle has size between 0° and 90°.

Obtuse angle :

Between 1/4 turn and 1/2 turn.

An obtuse angle has size between 90° and 180°.

Reflex angle :

Between 1/2 turn and 1 turn.

A reflex angle has size between 180° and 360°.

Vertically Opposite Angles :

Vertically opposite angles are formed when two straight lines intersect. The two angles are directly opposite each other through the vertex.

Transversal :

If a third line crosses two other straight lines we call it a transversal.

Corresponding Angles :

The angles marked . and "x" are corresponding angles because they are both in the same position. They are on the same side of the transversal and the same side of the two straight lines.

Alternate angle pair :

The angles marked . and x are alternate angles. They are on opposite sides of the transversal and between the two straight lines.

Co interior angle pair :

The angles marked . and x are co-interior angles. They are on the same side of the transversal and between the two straight lines. 

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