7-6 Vocabulary Dilation Scale factor

7-6 Dilations & Similarity in the Coordinate Plane Geometry

Identifying Dilations Previously, you studied rigid transformations, in which the image and preimage of a figure are congruent. In this lesson, you will study a type of nonrigid transformation called a dilation, in which the image and preimage of a figure are similar. A dilation with center C and scale factor k is a transformation that maps every point P in the plane to a point P so that the following properties are true. If P is not the center point C, then the image point P lies on . The scale factor k is a positive number such that k = , and k  1. CP CP  If P is the center point C, then P = P .

• • Identifying Dilations The dilation is a reduction if 0 < k < 1 and it is an enlargement if k > 1. P P´ 6 5 P´ P 3 2 Q´ Q • • R C C R´ Q R´ Q´ R Reduction: k = = = 3 6 1 2 CP CP Enlargement: k = = 5 2 CP CP Because PQR ~ P´Q´R´, is equal to the scale factor of the dilation. P´Q´ PQ

Ex. 1)If a photo is positioned at A(0,0), B(0,4), C(3,4), & D(3,0), then find the coordinates of the border of the photo after a dilation with scale factor of 5/2.

• Dilation in a Coordinate Plane In a coordinate plane, dilations whose centers are the origin have the property that the image of P(x, y) is P´(kx, ky). Draw a dilation of rectangle ABCD with A(2, 2), B(6, 2), C(6, 4), and D(2, 4). Use the origin as the center and use a scale factor of . How does the perimeter of the preimage compare to the perimeter of the image? 1 2 SOLUTION y Because the center of the dilation is the origin, you can find the image of each vertex by multiplying its coordinates by the scale factor. D C A(2, 2)  A´(1, 1) D´ A C´ B A´ B(6, 2)  B´(3, 1) 1 B´ • C(6, 4)  C ´(3, 2) O 1 x D(2, 4)  D´(1, 2)

Ex. 2 Given that find the coordinates of U & the scale factor. y S(0,16) U x R(12,0) T(-9, 0) 0

Ex. 3 Proving Triangles are Similar Given: E(-2,-6), F(-3,-2), G(2,-2), H(-4,2) & J(6,2). Prove:

Ex. 4) Graph the image of with A(0,3), B(3,6) & C(6,0) after a dilation with scale factor 2/3. Verify that

Assignment

• Identifying Dilations Identify the dilation and find its scale factor. • C P P´ 2 3 SOLUTION Because = , the scale factor is k = . 2 3 CP CP This is a reduction.

• • Identifying Dilations Identify the dilation and find its scale factor. • C P P´ 2 3 • P P´ C 1 2 SOLUTION SOLUTION Because = , the scale factor is k = . 2 3 CP CP Because = , the scale factor is k = 2. 2 1 CP CP This is a reduction. This is an enlargement.

• Dilation in a Coordinate Plane In a coordinate plane, dilations whose centers are the origin have the property that the image of P(x, y) is P´(kx, ky). Draw a dilation of rectangle ABCD with A(2, 2), B(6, 2), C(6, 4), and D(2, 4). Use the origin as the center and use a scale factor of . How does the perimeter of the preimage compare to the perimeter of the image? 1 2 SOLUTION y From the graph, you can see that the preimage has a perimeter of 12 and the image has a perimeter of 6. D C D´ A C´ B A preimage and its image after a dilation are similar figures. A´ 1 B´ • Therefore, the ratio of the perimeters of a preimage and its image is equal to the scale factor of the dilation. O 1 x

Using Dilations in Real Life Shadow puppets have been used in many countries for hundreds of years. A flat figure is held between a light and a screen. The audience on the other side of the screen sees the puppet’s shadow. The shadow is a dilation, or enlargement, of the shadow puppet. When looking at a cross sectional view, LCP ~ LSH. The shadow puppet shown is 12 inches tall (CP in the diagram). Find the height of the shadow, SH, for each distance from the screen. In each case, by what percent is the shadow larger than the puppet? LC = LP = 59 in.; LS = LH = 74 in. SOLUTION 59 74 12 SH = LC LS CP SH = 59 (SH) = 888 SH  15 inches

Using Dilations in Real Life Shadow puppets have been used in many countries for hundreds of years. A flat figure is held between a light and a screen. The audience on the other side of the screen sees the puppet’s shadow. The shadow is a dilation, or enlargement, of the shadow puppet. When looking at a cross sectional view, LCP ~ LSH. LC = LP = 59 in.; LS = LH = 74 in.; SH  15 inches SOLUTION To find the percent of size increase, use the scale factor of the dilation. scale factor = SH CP = 1.25 15 12 So, the shadow is 25 % larger than the puppet.

Using Dilations in Real Life Shadow puppets have been used in many countries for hundreds of years. A flat figure is held between a light and a screen. The audience on the other side of the screen sees the puppet’s shadow. The shadow is a dilation, or enlargement, of the shadow puppet. When looking at a cross sectional view, LCP ~ LSH. LC = LP = 66 in.; LS = LH = 74 in. SOLUTION 66 74 12 SH = LC LS CP SH = 66 (SH) = 888 SH  13.45 inches

Using Dilations in Real Life Shadow puppets have been used in many countries for hundreds of years. A flat figure is held between a light and a screen. The audience on the other side of the screen sees the puppet’s shadow. The shadow is a dilation, or enlargement, of the shadow puppet. When looking at a cross sectional view, LCP ~ LSH. LC = LP = 66 in.; LS = LH = 74 in.; SH  13.45 inches SOLUTION To find the percent of size increase, use the scale factor of the dilation. scale factor = SH CP = 1.12 13.45 12 So, the shadow is 12 % larger than the puppet.

Ex. 1 Identify the dilation & find its scale factor. C A B D