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11.04 Transformations

Lesson

Introduction

A transformation is when an object or point is moved, turned, flipped or changed in shape or size. We're going to consider a few types of transformations in which the shape and size of the image remain the same.

These transformations are known as:

  • translations

  • reflections

  • rotations

Multiple transformations can be combined in order to create a unique final image. The transformations are able to be reversed in order for us to find the object in its original position and orientation. Let's consider the three transformations in which the shape and size of the object remain the same.

Translations

A translation is a type of transformation for which the size, shape and rotation of the object being transformed does not change. We can think of sliding a shape across a page or screen as a translation.

Exploration

Play with the applet below by dragging the sliders to move the triangle around. Notice how the triangle does not change its orientation or size, it just slides from one place on the grid to another.

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When an object is being translated, its image moves without changing size or shape, rotating or reflecting.

When translating a shape, it can be helpful to choose a point on the shape which will be our reference point. To describe the translation between the original shape and the translated image, we can compare the corresponding reference points on both shapes. This allows us to easily find the horizontal and vertical distance between them.

If we were to choose two points on the original and translated image that do not match, our description of the translation will be incorrect.

In the figure below, triangle A has been translated 4 units to the right to produce triangle B. We can see this most easily by comparing the location of the bottom vertex of each triangle.

Triangle A with a width of 1 unit is moved 4 units to the right forming Triangle B. Ask your teacher for more information.

The translation from A to B is 4 units right.

Examples

Example 1

Plot the translation of the point by moving it 11 units to the left and 9 units down.

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Worked Solution
Apply the idea
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Move the point 11 units to the left, then 9 units down.

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We end up at the point (-3,-4).

Idea summary

A translation is a type of transformation for which the size, shape and rotation of the object being transformed does not change. We can think of sliding a shape across a page or screen as a translation.

Reflections

We can think of the reflection of an object or point as the original object being flipped over a line of reflection. This means that the original object and the reflected object have an equal perpendicular distance between the line of reflection. Note that the line of reflection does not have to be strictly horizontal or vertical and can be in any direction.

Exploration

Play with the following applet by dragging the sliders, and consider the way that the image moves relative to the object.

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The vertices of the triangle and its reflection have the same distance from the mirror line. The reflection is the mirror image of the original triangle on the other side of the line.

One way to think about this type of transformation is that we are flipping the object over the line of reflection to created the reflected image.

In the figure below, triangle A has been flipped over the line of reflection, resulting in triangle B. Notice that the corresponding points on A and B are equidistant from the line of reflection.

This image shows a triangle reflected across a line. Ask your teacher for more information.

Examples

Example 2

Consider the triangle below. Draw the result of the reflection across the line x=-1.

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Worked Solution
Create a strategy

The reflected points must have the same distance from the line of reflection.

Apply the idea

The point (-2, -1) is 1 unit away from x=-1, so the reflected point is (0,-1).

The point (-5, 9) is 4 units away from x=-1, so the reflected point is (3,9).

The point (-8, -5) is 7 units away from x=-1, so the reflected point is (6,-5).

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Idea summary

A reflection is when we flip an object or shape across a line. Like a mirror, the object is exactly the same size, just flipped in position. Every point on the object or shape has a corresponding point on the image, and they will both be the same distance from the reflection line.

Rotations

Another type of transformation, known as a rotation comes from rotating an image about a fixed point. The fixed point the image is rotated about is known as the centre of rotation.

Exploration

Play with the applet below to explore the rotation transformation. Try changing the shape and size of the original triangle, then use the slider to change the angle of rotation.

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The size and shape of the image of the object stays the same as it turns around the centre of rotation.

The centre of rotation does not always have to be a point on the image. Consider the figure below, which shows square A being rotated about the point O.

This image shows a rotation of a square. Ask your teacher for more information.

Square A is rotated 135\degree clockwise, or 225\degree anticlockwise, about O resulting in square B.

We can use a protractor to measure the angle of rotation between the original object and the rotated object. We can also use a protractor to measure the correct angle of rotation so we can draw the transformation.

Examples

Example 3

Consider the shape below. Draw the result of a rotation by 180\degree clockwise about point A.

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Worked Solution
Create a strategy

Draw horizontal and vertical line using A as the centre, then recall that the first quarter of rotation is 90\degree, second quarter is 180\degree, third quarter is 270\degree, and last quarter is 360\degree.

Apply the idea

180\degree is half of the full turn of 360 \degree, so the image should be halfway around a 360 \degree rotation from the original image.

The rotation is shown below.

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Idea summary

Rotation of a shape can be done by rotating it about a fixed point or the centre of rotation.

Outcomes

VCMMG261

Describe translations, reflections in an axis, and rotations of multiples of 90° on the Cartesian plane using coordinates. Identify line and rotational symmetries

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