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IM Geo.3.8 Lesson: Are They All Similar?

Are these rectangles similar? Explain how you know.

Tyler wrote a proof that all rectangles are similar. Make the image Tyler describes in each step in his proof. Which step makes a false assumption? Why is it false?

  • Draw 2 rectangles. Label one  and the other .
  • Translate rectangle  by the directed line segment from  to  and  now coincide. The points coincide because that’s how we defined our translation.
  • Rotate rectangle  by angle . Segment  now lies on ray . The rays coincide because that’s how we defined our rotation.
  • Dilate rectangle  using center  and scale factor . Segments  and  now coincide. The segments coincide because A'' was the center of the rotation, so  and  don’t move, and since  and  are on the same ray from , when we dilate  by the right scale factor, it will stay on ray  but be the same distance from  as  is, so  and  will coincide.
  • Because all angles of a rectangle are right angles, segment  now lies on ray . This is because the rays are on the same side of  and make the same angle with it. (If  and  don’t coincide, reflect across  so that the rays are on the same side of .)
  • Dilate rectangle  using center  and scale factor . Segments  and  now coincide by the same reasoning as in step 4.
  • Due to the symmetry of a rectangle, if 2 rectangles coincide on 2 sides, they must coincide on all sides.
  • Choose one statement from the list. Decide if it is true or not.

    Statements:

    1. All equilateral triangles are similar.
    2. All isosceles triangles are similar.
    3. All right triangles are similar.
    4. All circles are similar.
    If it is true, write a proof. If it is not, provide a counterexample.

    Repeat with another statement.

    Here is an x by x+1 rectangle and a 1 by x rectangle.

    They are similar. What are the possible dimensions of these golden rectangles? Explain or show your reasoning.