Understand transformations in the plane

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1

Consider Square . Perform two dilations successively, each with scale factor ; the first dilation should have center , the second, . Call the image of under these dilations ; the image of , , and so forth.

Which of the following diagrams correctly shows Square relative to Square ?

Squares

Squares

Squares

Squares

None of the other choices gives the correct response.

Explanation

To perform a dilation with center and scale factor , find the midpoints of the segments connecting to each point, and connect those points. We can simplify the process by finding the midpoints of , , and , and naming them , , and , respectively; , the image of center , is just itself. The figure is below:

Square 4

Now, do the same thing to the new square, but with as the center. The figure is below:

1

The final image, relative to the original square, is below:

Square 4

2

Nonagons

Examine the figures in the above diagram. Figure 2 is the result of performing which of the following transformations on Figure 1?

Explanation

The diagram below superimposes the two figures:

Nonagons

The transformation moves the black diagonal to the position of the red diagonal, and, consequently, points and to points and , respectively. This constitutes two-tenths of a complete turn clockwise, or a clockwise rotation of

3

Letters

Examine the figures in the above diagram. The figure at right is the result of performing which of the following transformations on the figure at left?

A clockwise rotation

A clockwise rotation

A counterclockwise rotation

A counterclockwise rotation

None of the other choices gives the correct response.

Explanation

Examine the figure below:

1

If we connect the horizontal line with the line along the rotated nine at right, we see that it is the result of a one-eighth turn clockwise; the angle between them .

4

Consider regular Hexagon ; let and be the midpoints of and . Reflect the hexagon about , then again about . Which of the following clockwise rotations about the center would result in each point being its own image under this series of transformations?

Explanation

Refer to the figure below, which shows the reflection of the given hexagon about ; we will call the image of , call the image of , and so forth.

Hexagon

Now, refer to the figure below, which shows the reflection of the image about ; we will call the image of , call the image of , and so forth.

Hexagon

Note that the vertices coincide with those of the original hexagon, and that the images of the points are in the same clockwise order as the original points. Since coincides with , coincides with , and so forth, a clockwise rotation of five-sixth of a complete turn - that is, ,

is required to make each point its own image under the three transformations.

5

Nonagons

Examine the figures in the above diagram. Figure 2 is the result of performing which of the following transformations on Figure 1?

Explanation

The diagram below superimposes the two figures:

Nonagons

The transformation moves the black diagonal to the position of the red diagonal, and, consequently, points and to points and , respectively. This constitutes two-tenths of a complete turn clockwise, or a clockwise rotation of

6

What is the result of rotating the point about the origin in the plane by ?

Explanation

Rotating a point

geometrically in the plane about the origin is equivalent to negating the coordinates of the point algebraically to obtain

.

Thus, since our initial point was

we negate both coordinates to get

as the rotation about the origin by .

7

Consider regular Hexagon ; let and be the midpoints of and . Reflect the hexagon about , then again about . With which of the following points does the image of under these reflections coincide?

Explanation

Refer to the figure below, which shows the reflection of about ; we will call this image .

Hexagon

Note that coincides with . Now, refer to the figure below, which shows the reflection of about ; we will call this image - the final image -

Hexagon

Note that coincides with , making this the correct response.

8

Letters

Examine the figures in the above diagram. The figure at right is the result of performing which of the following transformations on the figure at left?

A clockwise rotation

A clockwise rotation

A counterclockwise rotation

A counterclockwise rotation

A rotation

Explanation

Examine the figure below:

1

If we connect the horizontal line with the line along the rotated nine at right, we see that it is the result of a one-third turn clockwise; the angle between them

9

Letters

Examine the figures in the above diagram. The figure at right is the result of performing which of the following transformations on the figure at left?

A clockwise rotation

A clockwise rotation

A counterclockwise rotation

A counterclockwise rotation

None of the other choices gives the correct response.

Explanation

Examine the figure below:

1

If we connect the horizontal line with the line along the rotated nine at right, we see that it is the result of a one-eighth turn clockwise; the angle between them .

10

Letters

Examine the figures in the above diagram. The figure at right is the result of performing which of the following transformations on the figure at left?

A counterclockwise rotation

A clockwise rotation

A clockwise rotation

A counterclockwise rotation

A rotation

Explanation

Examine the figure below.

1

If we connect the horizontal line with the line along the rotated "omega" at right, we see that it is the result of a one-sixth turn counterclockwise; the angle between them is one sixth of , or .

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