A series of free, online High School Geometry Video Lessons.

In these lessons, we will learn

- Geometry Transformations and Isometries
- Geometry Translation
- Geometry Reflection
- Geometry Rotation
- Composition of Transformations

Related topics:

Videos, worksheets, and activities to help Geometry students.

### Geometry Transformations and Isometries

A transformation changes the size, shape, or position of a figure and creates a new figure. A geometry transformation is either rigid or non-rigid; another word for a rigid transformation is "isometry". An isometry, such as a rotation, translation, or reflection, does not change the size or shape of the figure. A dilation is not an isometry since it either shrinks or enlarges a figure.

**How to determine the types of transformations, the definition of isometry, and how to say and write a transformation's new image**
### Geometry Translation

A geometry translation is an isometric transformation, meaning that the original figure and the image are congruent.

Translating a figure can be thought of as "sliding" the original.

If the image moved left and down, the rule will be (x - __, y - __) where the blanks are the distances moved along each axis; for translations left and up: (x - __, y + __), for right and down (x + __, y - __), for right and up (x + __, y + __).

**How to perform a translation on the coordinate plane using a triangle?**
**How to translate a given polygon on the coordinate plane?**

### Geometry Reflection

A reflection is an isometry, which means the original and image are congruent, that can be described as a "flip". To perform a geometry reflection, a line of reflection is needed; the resulting orientation of the two figures are opposite. Corresponding parts of the figures are the same distance from the line of reflection. Ordered pair rules reflect over the x-axis: (x, -y), y-axis: (-x, y), line y = x: (y, x).

This video shows reflection over the

*x*-axis,

*y*-axis,

*x* = 2,

*y *= −2

This video shows reflection over

*y* =

*x, y = *−

*x*. A reflection that results in an overlapping shape.

This video shows reflection over the

*x*-axis,

*y*-axis,

*x* = −3,

*y *= 5,

*y* =

*x, *and

*y = *−

*x*.

### Geometry Rotation

A rotation is an isometric transformation: the original figure and the image are congruent. The orientation of the image also stays the same, unlike reflections. To perform a geometry rotation, we first need to know the point of rotation, the angle of rotation, and a direction (either clockwise or counterclockwise). A rotation is also the same as a composition of reflections over intersecting lines.

How to define a rotation and what information is necessary to perform a rotation.

The following videos show clockwise and anticlockwise rotation of 0˚, 90˚, 180˚ and 270˚ about the origin (0, 0). The pattern of the coordinates are also explored.

### Composition of Transformations

A composition of transformations is a combination of two or more transformations, each performed on the previous image. A composition of reflections over parallel lines has the same effect as a translation (twice the distance between the parallel lines). A composition of reflections over intersecting lines is the same as a rotation (twice the measure of the angle formed by the lines).

How to define a composition of transformations; how to perform a composition of translations and a composition of reflections.

Glide Reflection

A glide reflection is a composition of transformations. In a glide reflection, a translation is first performed on the figure, then it is reflected over a line. Therefore, the only required information is the translation rule and a line to reflect over. A common example of glide reflections is footsteps in the sand.

How to describe a glide reflection and identify the information needed to perform a glide reflection.
How to Apply Composition of Transformations.

Looking at composition of transformations - combining transformations in a series. Specifically looking at glide reflections, two reflections in parallel lines (a translation), and two reflections in intersecting lines (a rotation).

Two reflections in parallel lines is the same as a translation.

Two reflections in intersecting lines is the same as a rotation.

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