Geometry - Basics & Definitions Flashcards
Learn terminology and basics of lines, rays, planes, angles, trainees and quadrilaterals.
What is a plane?
In plane geometry, all the shapes exist in a flat plane. A plane can be thought of an a flat sheet with no thickness, and which goes on for ever in both directions. It is absolutely flat and infinitely large, which makes it hard to draw.
What is coplanar?
Two objects are coplanar if they both lie in the same plane
What is a point
A point is an exact position or location on a plane surface. It is important to understand that a point is not a thing, but a place. We indicate the position of a point by placing a dot with a pencil. This dot may have a diameter of, say, 0.2mm, but a point has no size. No matter how far you zoomed in, it would still have no width. Since a point is a place, not a thing, it has no dimensions.
What is a line?
A line is one-dimensional. It has zero width. If you draw a line with a pencil, examination with a microscope would show that the pencil mark has a measurable width. The pencil line is just a way to illustrate the idea on paper. In geometry however, a line has no width.
A straight line is the shortest distance between any two points on a plane.
What is a vertical line?
A vertical line is one which runs up and down the page.
What is a horizontal line?
In geometry, a horizontal line is one which runs from left to right across the page. It comes from the word ‘horizon’, in the sense that horizontal lines are parallel to the horizon.
What is a line segment?
A straight line which links two points without extending beyond them.
A line segment is one-dimensional. It has a measurable length, but has zero width. If you draw a line segment with a pencil, examination with a microscope would show that the pencil mark has a measurable width. The pencil line is just a way to illustrate the idea on paper. In geometry however, a line segment has no width.
What is midpoint of a line segment?
A point on a line segment that divides it into two equal parts
The halfway point of a line segment
Only a line segment can have a midpoint. A line cannot since it goes on indefinitely in both directions, and so has no midpoint. A ray cannot because it has only one end, and hence no midpoint.
What is a bisector?
A line, ray or segment which cuts another line segment into two equal parts.
In general ‘to bisect’ something means to cut it into two equal parts. The ‘bisector’ is the thing doing the cutting.
What is a Ray?
A portion of a line which starts at a point and goes off in a particular direction to infinity.
One way to think of a ray is a line with one end. A ray starts at a given point and goes off in a certain direction forever, to infinity. The point where the ray starts is called (confusingly) the endpoint.
A ray is one-dimensional. It has zero width. If you draw a ray with a pencil, examination with a microscope would show that the pencil mark has a measurable width. The pencil line is just a way to illustrate the idea on paper. In geometry however, a ray has no width.
A ray has no measurable length, because it goes on forever in one direction.
What is an Angle?
A shape, formed by two lines or rays diverging from a common point (the vertex).
An angle is a measure of turn. We measure angles in degrees . There are 360o in a full turn.
What is acute angle?
An angle whose measure is less than 90°
What is right angle?
An angle whose measure is exactly 90°
What is obtuse angle?
An angle whose measure is greater than 90° and less than 180°
What is a straight angle?
An angle whose measure is exactly 180° - a straight line
