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Lecture 1

MA 108 Lecture 1: Compositions of Transformations- Emily


Department
Mathematics
Course Code
MA 108
Professor
Elyse Suzanne Rogers
Lecture
1

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5) Compositions: When two or more transformations are combined to produce a single
transformation, the result is called a composition of the transformations. The composition of
two or more isometries is also an isometry. Compositions may preserve or reverse orientation
depending on how many reflections they contain.
An important type of composition is a glide reflection, which is a translation followed by
a reflection in a line parallel to the direction of the translation. A glide reflection is an isometry
that reverses the orientation of figures. For example, suppose a translation 6 units to the right is
followed by a reflection in the x-axis. This glide reflection can be written in coordinate form as
(x’,y’) = ( x + 6, -y ). The glide reflection ( x’, y’ ) = ( -x, y +4 ) is a translation 4 units up followed
by a reflection in the y-axis. The order in which two transformations are performed sometimes
affects the resulting image, but with a glide reflection the image would be the same regardless
of the order of the transformations (translation first or reflection first).
Try these: Write a brief description of each transformation:
1) ( x’, y’ ) = ( x - 3, -y ) a translation 3 units to the left, followed by a reflection in the x-axis.
2) ( x’, y’ ) = ( -x, y - 8 ) a translation 3 units down, followed by a reflection in the y-axis.
3) ( x’, y’ ) = ( y + 2, x + 2 ) a translation 2 units up, and 2 units to the right.
4) ( x’, y’ ) = ( x + 10, -y ) a translation 10 units to the right, followed by a reflection in the x-axis.
5) ( x, y ) = ( -x, y + 6 )a translation 6 units up, followed by a reflection in the y-axis.
Write each transformation in coordinate form:
6) Translation 5 units right and 5 units up, reflection in the line y = x. ( x, y ) = ( x+5, y+5) y,x
7) Translation 8 units left, reflection in the x-axis. ( x, y ) = ( x-8, -y )
8) Translation 5 units down, reflection in the y-axis. ( x, y ) = (- x, y-5 )
9) Translation 4 units right, reflection in the x-axis.( x, y ) = (x+4, -y)
Some transformations can be thought of as a composition of two reflections. A
translation is the composition of two reflections in parallel lines. The direction of the translation
is perpendicular to the lines, and the distance translated is twice the distance between the lines.
A rotation can be expressed as the composition of two reflections in intersecting lines. The
center of the rotation is the point where the lines intersect, and the angle of the rotation is twice
the angle between the lines.
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