Grade 3/Geometry
Find the Missing Side from Perimeter: Multiple Choice (711)
Use the perimeter formula to solve for the unknown side length of rectangles.
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Teacher Resources
Standards
CCSS.MATH.CONTENT.3.MD.C.5Teaching Notes
Emphasize using the perimeter formula P=2(L+W). Guide learners through inverse operations to isolate the unknown side.
Vocabulary
Perimeter: The total distance around a shape.
Side length: The measurement of one edge of a shape.
Common Mistakes
- Forgetting to divide the perimeter by 2 first
- Subtracting the known side twice
- Mixing up which side is missing
- Forgetting to divide perimeter by two
- Calculation errors during subtraction
- Mixing up length and width
Differentiation
SupportUse manipulatives or drawings to visualize rectangles.
ChallengeIntroduce word problems or non-rectangular shapes.
Discussion Questions
- What is the definition of perimeter?
- Why do we divide the perimeter by two?
- How is this skill useful in real life?
- What if the shape was a square?
Extension Activities
- Draw different rectangles with the same perimeter.
- Find missing sides of irregular shapes.
Parent Tip
Walk around your house and count steps to estimate perimeter.
Learning Path
Skill Cluster
Geometry and Measurement
Estimated Time
14 minutes
Skills Practiced
Solving for an unknown side using perimeterWriting and solving two-step equationsChecking whether answers are reasonable
Prerequisites
- Perimeter of rectangles (formula)
- Understanding perimeter
- Addition and Subtraction
- Multiplication and Division facts
Next Steps
- Finding missing side with decimals
- Perimeter of irregular polygons
- Area of rectangles
- Calculate Perimeter of Rectangles
- Area of Rectangles
- Perimeter Word Problems