Title: Lesson 1–1
Author: Teri Willard
Last saved by: MHE

Lesson 4–4

Example 1  Translate a Figure
Find the coordinates of the vertices of the image of quadrilateral ABCD with A(–4, 1), B(–2, 3), 
C(0, 0), and D(–4, –3), if it is moved 5 units to the right and 3 units down. Then graph ABCD and its image A'B'C'D'.

Write the vertex matrix for quadrilateral ABCD.

To translate the quadrilateral 5 units to the right, add 5 to each x-coordinate. To translate the figure 3 units down, add –3 to each y-coordinate. This can be done by adding the translation matrix  to the vertex matrix of ABCD.

vertex matrix 
of ABCD

translation 
matrix

vertex matrix
of A'B'C'D'

| + | | = |
	
The coordinates of A'B'C'D' are A'(1, –2), 
B'(3, 0), C'(5, –3), D'(1, –6). ABCD and A'B'C'D' have the same size and shape.

Example 2  Find a Translation Matrix
Short Response Test Item
X'Y'Z' is the result of a translation of XYZ. A table of the translations is shown. Find the coordinates of Z'. 

XYZ | X'Y'Z'
X(–3, 2) | X'(4, 7)
Y(-4, -5) | Y'(3, 0)
Z(2, –1) | Z'
A (-3, -8) 	B (7, 6)	C (-5, -6)	D (9, 4)

Read the Test Item
You are given the coordinates of the preimage and image of points X and Y. Use this information to find the translation matrix. Then you can use the translation matrix to find the coordinates of Z'.

Solve the Test Item
Step 1   Write a matrix equation. Let (c, d) represent the coordinates of Z'. 
+ | = |
| = |

Step 2   Since these two matrices are equal, corresponding elements are equal. 
| Solve an equation for x. | | Solve an equation for y.
| –3 + x = 4 | | 2 + y = 7
| x = 7 | | y = 5

Step 3   Use the values for x and y to find the values for Z' (c, d).
| 2 + x = c | | –1 + y = d
| 2 + 7 = c | | –1 + 5 = d
| 9 = c | | 4 = d

The coordinates for Z' are (9, 4), and the answer is D.

Example 3  Dilation
Dilate quadrilateral DEFG with vertices D(1, 2), E(4, 1), F(3, –1), and G(0, 0) so that its perimeter is two and one–half times the original perimeter. Find the coordinates of the vertices of quadrilateral D'E'F'G'.

If the perimeter of a figure is two and one–half times the original perimeter, then the lengths of the sides of the figure will be two and one–half times the measure of the original lengths.

Multiply the vertex matrix by the scale factor of 2.5.

2.5  = 

The coordinates of the vertices of quadrilateral D'E'F'G' are D'(2.5, 5), and E'(10, 2.5), F'(7.5, –2.5), 
and G'(0, 0).

Example 4  Reflection
Find the coordinates of the vertices of the image of quadrilateral ABCD with A(–2, 1), B(–1, 4), 
C(3, 2), and D(4, –2) after a reflection across the line y = x. 

Write the ordered pairs as a vertex matrix. Then multiply the vertex matrix by the reflection matrix for the line y = x.

   = 

The coordinates of the vertices of A'B'C'D' are A'(1, –2), B'(4, –1), C'(2, 3), and D'(–2, 4). Notice that the preimage and image are congruent. Both figures have the same size and shape.

Example 5  Rotation
Find the coordinates of the vertices of the image of quadrilateral MNOP with M(2, 2), N(2, 5), 
O(3, 4), and P(4, 1) after it is rotated 270˚ counterclockwise about the origin.

Write the ordered pairs in a vertex matrix. Then multiply the vertex matrix by the rotation matrix.
	
   = 

The coordinates of the vertices of quadrilateral M'N'O'P' are M'(2, –2), N'(5, –2), O'(4, –3), and P'(1, –4). The image is congruent to the preimage.