Lesson 8
Ways to Find Unknown Length (Part 2)
Lesson Purpose
Lesson Narrative
In the previous lesson, students used given side lengths and symmetry to find the perimeter of a figure. In this lesson, students reason in the other direction—given the perimeter and information about symmetry, they find the side lengths of twodimensional figures. Students also practice completing a figure given a line of symmetry and half of the figure, and then reason about the perimeter of the whole figure. Along the way, students reinforce their ability to add fractions and to multiply fractions by whole numbers.
 Engagement
 MLR8
Activity 2: Lin’s Design
Learning Goals
Teacher Facing
 Find the unknown side lengths of twodimensional shapes using their attributes.
Student Facing
 Let’s find the unknown lengths in figures.
Required Materials
Materials to Gather
Required Preparation
CCSS Standards
Addressing
Lesson Timeline
Warmup  10 min 
Activity 1  20 min 
Activity 2  15 min 
Lesson Synthesis  10 min 
Cooldown  5 min 
Teacher Reflection Questions
The tasks in this lesson prompted students to apply what they learned about adding fractions and multiplying fractions by a whole number. How comfortable were students with these operations? What ideas or strategies do students need more practice with?
Suggested Centers
 Which One? (K–5), Stage 5: Grade 4 Shapes (Addressing)
 Can You Draw It? (1–5), Stage 5: Grade 4 Shapes (Addressing)
 Compare (1–5), Stage 5: Fractions (Supporting)
 Compare (1–5), Stage 7: Multidigit Operations (Supporting)
Print Formatted Materials
For access, consult one of our IM Certified Partners.
Additional Resources
Google Slides  For access, consult one of our IM Certified Partners. 

PowerPoint Slides  For access, consult one of our IM Certified Partners. 