Lesson 6: Similarity

Let’s explore similar figures.

6.1: Equivalent Expressions

Use what you know about operations and their properties to write three expressions equivalent to the expression shown.

10 (2 + 3) - 8 \boldcdot 3

6.2: Similarity Transformations (Part 1)

  1. Triangle EGH and triangle LME are similar. Find a sequence of translations, rotations, reflections, and dilations that shows this.

    GeoGebra Applet b25xp66d

     

  2. Hexagon ABCDEF and hexagon HGLKJI are similar. Find a sequence of translations, rotations, reflections, and dilations that shows this.

    GeoGebra Applet U7wJUpeD

     

6.3: Similarity Transformations (Part 2)

Sketch figures similar to Figure A that use only the transformations listed to show similarity.

  1. A translation and a reflection. Label your sketch Figure B. Pause here so that your teacher can check your work.
  2. A reflection and a dilation with scale factor greater than 1. Label your sketch Figure C.
  3. A rotation and a reflection. Label your sketch Figure D.
  4. A dilation with scale factor less than 1 and a translation. Label your sketch Figure E.

6.4: Methods for Translations and Dilations

Your teacher will give you a set of five cards and your partner a different set of five cards. Using only the cards you were given, find at least one way to show that triangle ABC and triangle DEF are similar. Compare your method with your partner’s method. What is the same about your methods? What is different?

 

Summary

Let’s show that triangle ABC is similar to triangle DEF:

Two figures are similar if one figure can be transformed into the other by a sequence of translations, rotations, reflections, and dilations. There are many correct sequences of transformations, but we only need to describe one to show that two figures are similar.

One way to get from ABC to DEF follows these steps:

  • step 1: reflect across line f
  • step 2: rotate 90^\circ counterclockwise around D
  • step 3: dilate with center D and scale factor 2

Another way would be to dilate triangle ABC by a scale factor of 2 with center of dilation A, then translate A to D, then reflect over a vertical line through D, and finally rotate it so it matches up with triangle DEF. What steps would you choose to show the two triangles are similar?

Practice Problems ▶

Glossary

similar

similar

One figure is similar to another if there is a sequence of rigid transformations and dilations that moves the first figure so that it fits exactly over the second.

Triangle ABC is similar to triangle DEF because a rotation about B followed by a dilation with center O takes the first triangle to the second.