7.11 The student will determine if geometric figures - quadrilaterals and triangles - are similar and write proportions to express the relationships between corresponding parts of similar figures.


  • 1. Look at the two different triangles, red and green- are these triangles similar?

  • 2. Note that the parameter of each triangle is listed, along with a slider to control the length of the base side. What are the values of the triangle perimeters, and what is the relationship of these two triangle perimeters to each other?

  • 3. If we manipulate the slider to change one triangles side and not the other- what happens to the relationship of these two triangles?

  • 4. Does the similarity between these two triangles and their respective perimeters have any relationship to each other?

  • 5. Does the relationship between the two triangle perimeters decide if these triangles are similar to each other?
  • Theorem

  • Proportional Perimeter Theorem- If two triangles are similar, then the perimeters are proportional to the measures of the corresponding sides.
  • Hands-On Activities

    • Activity 1
    • Activity 2
    • Regentsprep Explore similar figures and practice your skills.

    Other Web Sites

    • Mathleague View examples of similar figures.
    • ExploreLearning Teachers have to assign the Similar Figures-Activity B to their classes.
    • Larson´s Software Students need to be enrolled by their teachers. Click on the More Geometry in a Plane, then click on Congruent and Similar Figures.

    Puzzles and Problems

    • River Problem See help in the 2nd Video Notes
    • Great Cartoon Blow-up Students use cartoon pictures and draw an enlarged, similar version of them. They must first use proportions to determine the correct size.

    Lesson 4-3: Similar Figures

    See the direction on the right. Answer the questions using the GeoGebra Interactive object below to test out your ideas: Sorry, the GeoGebra Applet could not be started. Please make sure that Java 1.4.2 (or later) is installed and activated. (click here to install Java now)

    Created with GeoGebra by Norm Ebsary, Fall 2006.