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The Line and Transformations

Topic Overview
 
Question 3 on Paper 2 deals with both of these topics, which are connected in that they both deal with the Cartesian plane. The Cartesian, or coordinated, plane was first introduced by the French mathematician, Decartes, early in the 17th century. This was a major leap forward, as it allowed, for the first time, the power of algebra to be used for dealing with problems in geometry, and, conversely, a geometric interpretation to be put on many algebraic problems. It was this latter use that fascinated Descartes, but our concern nowadays is more with the former.

By the time students start studying Leaving Cert maths, they have normally been dealing with co-ordinate geometry for two to three years. This familiarity leaves most well disposed towards The Line. On this course, the basic formulae and methods from Junior Certificate, e.g., distance, midpoint, slope, equation of a line, area of a triangle, are augmented by a few new formulae and methods. Whereas these command most attention, the more basic ideas should not be neglected.

Transformations, or linear transformations to give them their full title, are functions which map points in the plane to other points in the plane. However, they do so in a very regular and organised way, so that certain properties are always preserved. For example, lines are always mapped to lines, and parallel lines are always mapped to parallel lines. But other properties, e.g., distance, are not necessarily preserved. On our course, we investigate, for specific transformations, such features of points, lines and shapes in the plane under the action of a transformation.

Topic Structure: The Line
 
The study of The Line for Leaving Cert can be broken down into the following sections:

1. Basic Co-ordinate Geometry Results
Standard Junior Cert. formula
It doesn't matter which point is which
The slope, or gradient, is a measure of the steepness of the line
To use the first form, we need a point and a slope
Translate other triangles so that the origin is a vertex
We find the components of the translation

2. Leaving Cert Formulae
Be careful to get the points and the ratio in the right order
We can use the modulus to get the smaller angle
We use the other given fact to find lambda
Also called the shortest distance from a point to a line

3. Leaving Cert Methods
Let each point in the locus be (x,y)
Used when the information given makes it difficult to get a point and a slope

Topic Structure: Transformations

The study of Leaving Cert Transformations can be broken down into the following sections:

1. Methods
Substitute the coordinates for x and y
Solve simultaneous equations, or use matrices
Substitute for x and y in the equation of the line
Be careful to limit the values to confine the argument to a line segment

2. Proofs and Investigations
The second part of the proof involves showing that the image is ALL of a line
Be careful to limit the values
Generally we find the image lines and show that they have the same slope
This follows directly from the previous properties
Simply find the images of points or lines and check as required

Links
http://www.ping.be/math/lintf.htm
This 'Linear Transformations' section has a slightly different emphasis than what else appears in the study notes, but interested students will find it useful and perhaps gain additional insight into familiar material.

http://wims.unice.fr/~wims/wims.cgi?session=Q15D9FA7A4.2&lang=en&module=H4%2Fgeometry%2Flinechoice.en
This excellent link is to an interactive exercise in the equations of lines, converting from one form to another, including parametric, and plotting and recognising lines.

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