# Lesson 2 Triangle Dilations Solidify Understanding

## Jump Start

Given

## Learning Focus

Create similar figures by dilation given the scale factor.

Prove a theorem about the midlines of a triangle using dilations.

How do I know if two geometric figures are similar?

How do I know if two geometric figures are similar by dilation?

What interesting characteristics of an image are produced by dilating a polygon centered at one of the vertices of the polygon?

## Open Up the Math: Launch, Explore, Discuss

### 1.

Given

, use point as the center of a dilation with a scale factor of to locate the vertices of a new triangle, . Now use point

as the center of a dilation with a scale factor of to locate the vertices of a new triangle, .

### 2.

Label the vertices in the two triangles you created in the diagram above. Based on this diagram, write several proportionality statements you believe are true. First write your proportionality statements using the names of the sides of the triangles in your ratios. Then verify that the proportions are true by replacing the side names with their measurements, measured to the nearest millimeter. (Try to find at least 10 proportionality statements you believe are true.)

My list of proportions:

### 3.

Based on your work, under what conditions are the corresponding line segments in an image and its pre-image parallel after a dilation? That is, which word best completes this statement:

After a dilation, corresponding line segments in an image and its pre-image are parallel.

#### A.

never

#### B.

sometimes

#### C.

always

### 4.

Give reasons for your answer. If you choose “sometimes,” be very clear in your explanation how to tell when the corresponding line segments before and after the dilation are parallel and when they are not.

### 5.

Given

### 6.

Explain how the diagram you created above can be used to prove the following theorem:

*The segment joining midpoints of two sides of a triangle is parallel to the third side and half the length.*

## Ready for More?

In problem 1,

#### a.

#### b.

What is the scale factor for this new dilation?

## Takeaways

Key relationships between an image and its pre-image after a dilation:

## Adding Notation, Vocabulary, and Conventions

A dilation is a transformation of the plane, such that if

A two-dimensional figure is similar to another if

## Vocabulary

- midline of a triangle
- midline of a triangle theorem
**Bold**terms are new in this lesson.

## Lesson Summary

In this lesson, we extended our understanding of similar figures. Since corresponding segments of similar figures are proportional, and dilations produce similar figures, corresponding parts of an image and its pre-image after a dilation are proportional. We also learned that corresponding line segments in a dilation are parallel. These two observations provided a tool for proving a theorem about the midlines of a triangle, a segment connecting the midpoints of two sides of a triangle.

### 1.

Match the diagrams with the angle type.

___

___

___

Alternate Interior

Linear Pair

Vertical Angles

Same Side Exterior

Corresponding

Alternate Exterior

Complementary

Same Side Interior

### 2.

Describe the transformation(s) that occurred between the pre-image and image.

### 3.

Describe the transformation(s) that occurred between the pre-image and image.