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Reflections

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What if you were given the coordinates of a quadrilateral and you were asked to reflect that quadrilateral over the y- axis? What would its new coordinates be? After completing this Concept, you'll be able to reflect a figure like this one in the coordinate plane.

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Transformation: Reflection CK-12

Guidance

A transformation is an operation that moves, flips, or otherwise changes a figure to create a new figure. A rigid transformation (also known as an isometry or congruence transformation ) is a transformation that does not change the size or shape of a figure.

The rigid transformations are Transformation: Translation , reflections (discussed here), and Transformation: Rotation . The new figure created by a transformation is called the image . The original figure is called the preimage . If the preimage is A , then the image would be A' , said “a prime.” If there is an image of A' , that would be labeled A'' , said “a double prime.”

A reflection is a transformation that turns a figure into its mirror image by flipping it over a line. The line of reflection is the line that a figure is reflected over. If a point is on the line of reflection then the image is the same as the preimage. Images are always congruent to preimages.

While you can reflect over any line, some common lines of reflection have rules that are worth memorizing:

Reflection over the y- axis: (x,y) \rightarrow (-x,y)

Reflection over the x- axis: (x,y) \rightarrow (x,-y)

Reflection over y = x : (x,y)\rightarrow(y,x)

Reflection over y = -x : (x,y) \rightarrow (-y,-x)

Example A

Reflect \triangle ABC over the y- axis. Find the coordinates of the image.

\triangle A'B'C' will be the same distance away from the y- axis as \triangle ABC , but on the other side. Hence, their x -coordinates will be opposite.

& A(4,3) \rightarrow A'(-4,3)\\& B(7,-1) \rightarrow B'(-7,-1)\\& C(2,-2) \rightarrow C'(-2,-2)

Example B

Reflect the letter ``F'' over the x- axis.

When reflecting the letter F over the x- axis, the y- coordinates will be the same distance away from the x- axis, but on the other side of the x- axis. Hence, their y -coordinates will be opposite.

Example C

Reflect the triangle \triangle ABC with vertices A(4, 5), B(7, 1) and C(9, 6) over the line x = 5 . Find the coordinates of A' , B' , and C' .

The image’s vertices are the same distance away from x = 5 as those of the preimage.

& A(4,5) \rightarrow A'(6,5)\\& B(7,1) \rightarrow B'(3,1)\\& C(9,6) \rightarrow C'(1,6)

Transformation: Reflection CK-12

Guided Practice

1. Reflect the line segment \overline{PQ} with endpoints P(-1, 5) and Q(7, 8) over the line y = 5 .

2. A triangle \triangle LMN and its reflection, \triangle L'M'N' are below. What is the line of reflection?

3. Reflect square ABCD over the line y = x .

4. Reflect the trapezoid TRAP over the line y = -x .

Answers:

1. P is on the line of reflection, which means P' has the same coordinates. Q' is the same distance away from y = 5 , but on the other side.

P(-1,5) & \rightarrow P'(-1,5)\\Q(7,8) & \rightarrow Q'(7,2)

2. Looking at the graph, we see that the corresponding parts of the preimage and image intersect when y = 1 . Therefore, this is the line of reflection.

If the image does not intersect the preimage, find the midpoint between the preimage point and its image. This point is on the line of reflection.

3. The purple line is y = x . Fold the graph on the line of reflection.

& A(-1,5) \rightarrow A'(5,-1)\\& B(0,2) \rightarrow B'(2,0)\\& C(-3,1) \rightarrow C'(1,-3)\\& D(-4,4) \rightarrow D'(4,-4)

4. The purple line is y = -x . You can reflect the trapezoid over this line.

& T(2,2) \rightarrow T'(-2,-2)\\& R(4,3) \rightarrow R'(-3,-4)\\& A(5,1) \rightarrow A'(-1,-5)\\& P(1,-1) \rightarrow P'(1,-1)

Practice

  1. If (5, 3) is reflected over the y- axis, what is the image?
  2. If (5, 3) is reflected over the x- axis, what is the image?
  3. If (5, 3) is reflected over y = x , what is the image?
  4. If (5, 3) is reflected over y = -x , what is the image?
  5. Plot the four images. What shape do they make? Be specific.
  6. Which letter is a reflection over a vertical line of the letter ``b'' ?
  7. Which letter is a reflection over a horizontal line of the letter ``b'' ?

Reflect each shape over the given line.

  1. y- axis
  2. x- axis
  3. y = 3
  4. x = -1
  5. x- axis
  6. y- axis
  7. y = x
  8. y = -x
  9. x = 2
  10. y = -4
  11. y = -x
  12. y = x

Find the line of reflection the blue triangle (preimage) and the red triangle (image).

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