This Instructional Activity is a part of the Lesson 1, Unit 3, Grade 8. In mathematics, we want to be absolutely sure about what we are saying. Therefore, we need precise definitions for similar figures. For example, you may have thought that the figures in Pair G are similar because they are both happy faces. However, a precise definition of similarity tells you that they are in fact not similar because the parts of the face are not in proportion. Think about trying to adjust the size of a digital picture.