Boundary Builders Teacher Guide Boundary Builders
Teacher Facilitation Guide | Grade 3 Tier 2 Intervention
Standard CO 3.MD.D.8
Learning Objectives
Students will define perimeter as the distance around a polygon.
Students will calculate perimeter by adding all side lengths.
Students will find an unknown side length when given the total perimeter.
Materials Needed
• Pre-cut lengths of string
• Rulers (cm/inches)
• Geoboards & rubber bands
• "Fence Finder" Activity Sheet
• "Geoboard Blueprints" Cards
Phase 1: Concrete Exploration (10-15 mins)
Focus: Building the concept of "Around".
Prompting Questions:
"If an ant wanted to walk all the way around the edge of this shape, how far would it go?"
Give students the Fence Finder worksheet and a piece of string.
Model laying the string exactly along the border of the shape.
Mark the end of the string, then stretch it out against a ruler to find the "Total Distance Around".
Teacher Note: Emphasize that we don't care about the space inside (area), only the "fence" around it.
Phase 2: Building Boundaries (15 mins)
Focus: Unit segments and simple addition.
Students use geoboards to build the shapes on the Geoboard Blueprints task cards.
Teach students to count the units between pegs as side lengths.
Model the "Touch and Add" strategy: touch each side, say its length, and write the addition sentence: \(P = 3 + 4 + 3 + 4\).
Phase 3: The Missing Link (10 mins)
Focus: Problem solving with missing sides.
The "Hiding Side" Script:
"We know the whole fence is 20 feet. We have these three sides: 5, 5, and 4. How much more do we need to reach 20?"
Teach the 2-step process:
Step 1: Add the sides you know.Step 2: Subtract that sum from the Total Perimeter.
Progress Monitoring
Observation Look-Fors:
• Does student count units correctly on geoboard?
• Do they skip sides when adding?
• Can they explain perimeter in their own words?
Check-In Questions:
• "Where is the perimeter on this desk?"
• "If I add another side, what happens to the perimeter?"
Perimeter Power Slides Boundary Builders
Mastering the Art of Perimeter
What is Perimeter?
Perimeter is the total distance around the outside of a shape.
Think of it like a fence around a yard or the edge of a picture frame.
Around
The Addition Strategy
1
Measure every side.
2
Add all the lengths together.
P = s1 + s2 + s3 + s4
8 cm 8 cm 6 cm
8 + 8 + 6 = 22 cm
The Missing Side Mystery
Farmer Ben is building a fence for his goats. The Total Perimeter must be 30 feet . He has finished 3 sides: 10 ft, 5 ft, and 10 ft.
Step 1: Add Known Sides
10 + 5 + 10 = 25
Step 2: Find the Difference
30 - 25 = 5
The missing side is 5 feet!
Time to Build!
Grab your Geoboard and Rubber Bands . We are going to build polygons and calculate their perimeters together!
1. BUILD IT
2. COUNT IT
3. ADD IT
Fence Finder Activity Fence Finder
Measure the distance around each "field"!
Name: ____________________________
Date: _____________________________
How to Play:
Lay your string exactly on the dotted line of the shape.
Mark where the string meets to complete the loop.
Stretch the string out against your ruler .
Write the total length (perimeter) in the box!
Field A
Perimeter A:
Field B
Perimeter B:
Field C
Perimeter C:
Field D
Perimeter D:
Think About It:
Which field has the longest fence? How can you tell just by looking?
Geoboard Blueprints Task Cards Geoboard Blueprints
Build it on your geoboard, count the units, and find the perimeter!
CARD #1
Build a rectangle that is 4 units long and 2 units wide.
Calculation: ____ + ____ + ____ + ____
Perimeter: __________ units
CARD #2
Build a triangle where each side is 3 units long.
Calculation: ____ + ____ + ____
Perimeter: __________ units
CARD #3
Build a rectangle with a Total Perimeter of 12.
Hint: Try making two sides 4 units long!
My Sides: ____ , ____ , ____ , ____
Total: 12 units
CARD #4
Build an L-shape that has a Perimeter of 10.
Count the units around carefully!
Check: Does it add to 10?
Perimeter Patrol Check Perimeter Patrol
Progress Monitoring Check
Officer:
Date:
1
What is the perimeter of this polygon?
Show your addition sentence.
7 cm 7 cm 4 cm 4 cm
Show Work:
Answer: ________ cm
2
Find the missing side length.
Total Perimeter = 24 inches
8 in 8 in ?
Steps:
1. Add known sides: ____________
2. Subtract from 24: ____________
Missing Side: ________ in
3
Real-World Problem
Sarah wants to put a lace border around her square napkin. Each side of the napkin is 6 inches long. How much lace does she need in total?
Drawing Space / Work Area
Total Lace: ________ inches
Boundary Builders Answer Key Answer Key
Teacher Reference Sheet
Material: Fence Finder Activity
Field A (Rectangle): ~ 30-32 cm (varies by print)
Field B (Square): ~ 24-26 cm
Field C (Triangle): ~ 28-30 cm
Field D (L-Shape): ~ 34-36 cm
*Note: Actual measurements depend on printer scaling settings (Actual Size vs. Fit to Page).*
Material: Geoboard Blueprints
CARD 1
Calculation: 4 + 2 + 4 + 2 | Perimeter: 12 units
CARD 2
Calculation: 3 + 3 + 3 | Perimeter: 9 units
CARD 3
Possible Sides: 4, 2, 4, 2 (or 3, 3, 3, 3 or 5, 1, 5, 1) | Total: 12
CARD 4
L-Shape Check: Ensure student counts all 6 sides of the L-shape. | Total: 10
Material: Perimeter Patrol Check
1. Rectangle Perimeter
Work: 7 + 4 + 7 + 4 = 22
Answer: 22 cm
2. Missing Side Triangle
Step 1: 8 + 8 = 16
Step 2: 24 - 16 = 8
Answer: 8 inches
3. Sarah's Napkin (Square)
Square sides are all equal. 6 + 6 + 6 + 6 = 24
Answer: 24 inches
Intervention Tip:
If students get 12 for the square napkin (Question 3), they likely only added two sides. Remind them a square has four sides and a fence must go all the way around!
