1. Lesson 14.5 Pre-AP Geometry
Dilations
Lesson 14.5
Pre-AP Geometry

2. Lesson Focus
There are transformations that do not preserve distance. This lesson introduces one such transformation, called a dilation.

3. Basic Terms
Dilation A transformation that changes the size of a figure by a scale factor to create a similar figure. A dilation is not rigid. DO,k A dilation with a center O and a nonzero scale factor k maps any point P to a point P’. 1) If k > 0, P’ lies on and OP’ = k · OP. 2) If k < 0, P’ lies on the ray opposite and OP’ = |k| · OP.

4. Basic Terms
Expansion A dilation where the scale factor |k| > 1. Contraction A dilation where the scale factor |k| < 1.

5. Basic Terms
Similarity Mapping A transformation that maps any geometric figure to a similar geometric figure. A dilation is not an isometry as distances are not preserved.

6. Theorem 14-5
A dilation maps a triangle to a similar triangle.

7. Corollary 1
A dilation maps an angle to a congruent angle.

8. Corollary 2
A dilation DO,k maps any segment to a parallel segment |k| times as long.

9. Corollary 3
A dilation DO,k maps any polygon to a similar polygon whose area is k2 times as large.

10. Practice Given: A(3, 6), B(-3, -3), and C(-6, 0).
Find: (a) DO,2 ; (b) DO, -1/3
2. A dilation with the origin, O, as center maps (3, 4) to (9, 12). Find the scale factor. Is the dilation an expansion or a contraction?
A dilation with the origin, O, as center maps
(-3, 4) → (1, -4/3). Find the scale factor.
Is the dilation an expansion or contraction?

11. Review Which transformations are isometries?
If g(x) = 7 – 2x, find the image of 3 and the preimage of -5.
If R: (x, y) → (x – 2, y + 3), find the image of (-3, 1).
Find the image of (-1, 4) when reflected in each line.
a. the x-axis
b. the y-axis
c. the line y = x

12. Problem Set 14.5, p. 596: # 2 – 8 (even) Handout: 14-5
Written Exercises
Problem Set 14.5, p. 596: # 2 – 8 (even) Handout: 14-5