Subject: Geometry

# Words & Symbols In Geometry

This section will serve as a basic introduction to some of the things youĺll see in geometry ľ youĺll no doubt have heard many of these terms and seen many of these symbols before, but itĺs good to see what weĺll be using, and itĺs handy to have a quick guide.

### A List of Words Used in Geometry

(More in depth explanations and examples will be covered in subsequent pages!)

 The Breakdown

## POINTS, LINES AND PLANES

Line
In geometry, the definition of line must be more precise than our everyday use of the word because we also have to deal with rays and segments. A line may be straight or curved, and straight lines continue on past any points upon them. Lines are usually named after their two furthest points.
Ray
A portion of a straight line that begins at a point and continues indefinitely.
Segment
A portion of a straight line with two distinct endpoints, a line with finite length.
Point
A symbol used to show position. Represented by a small dot or a letter.
Endpoint
A point at one end of a segment, or the beginning of a ray.
Midpoint
A point exactly halfway along a segment.
Axis / Axes
A fixed, numbered reference line used in coordinate geometry.
Origin
The point at which two or more axes meet in coordinate geometry.
Vertex
The point where two straight lines, segments or rays meet creating an angle.
Bisect
To bisect is to cross a line or segment at itĺs midpoint.
Plane
A two-dimensional form with length and width but no depth or thickness.
Collinear
A set of two or more points is said to be collinear if they lie upon the same line.
Congruent
Segments that have the same length are congruent.

## ANGLES

Measure
A word used when talking about the value of an angle. An example might be ôThe measure of angle X is 120░ö.
Right Angle
An angle with a measure of exactly 90░.
Perpendicular
Two lines are perpendicular if the angle which their intersection or meeting point forms is exactly 90░.
Obtuse
An angle with measure greater than 90░ and less than 180░.
Acute
An angle with a measure of less than 90░.
Two angles are adjacent if they share the same vertex.
Vertical Angles
Two non-adjacent angles formed by intersecting lines.
Complementary
Angles whose combined total value add to exactly 90░.
Supplementary
Angles whose combined total value add to exactly 180░.

## CIRCLES

Chord
A straight line joining any two points on a circle.
Secant
A straight line joining any two points on a circle and continuing beyond those points in both directions.
Arc
A portion of the circumference of a circle between and including two points.
Diameter
A chord that passes through the middle of the circle.
The straight length or distance between the centre of a circle and a point on its circumference. The radius of a circle is exactly half of that circles diameter. The plural of radius is radii (pronounced Ĺray-dee-eyeĺ).
Circumference
The length around a circle.
Semicircle
An arc measuring exactly half the circumference of a circle. Literally half a circle.
Central Angle
An angle formed by two radii of the same circle (not two overlapping circles).
Pi
A constant, irrational number derived from the ratio of the circumference of a circle to its diameter. Pi always has the same value, regardless of the size of a circle. This value has been calculated to many trillions of digits, but 3.141 is fine for our purposes.

## OTHER

Parallelogram
An enclosed two-dimensional shape wherein all opposing sides are parallel.
Polygon
An enclosed two-dimensional shape with a finite number of straight sides.