LESSON PLAN

Perimeter of Regular and Irregular Shapes

Apothem Team

Grade 4 · Measurement

LESSON AT A GLANCE

Warm-up

5 min

Explore

15 min

Consolidate

10 min

Practice

12 min

Exit ticket

3 min

Warm-up

On a geoboard, make a shape and count the elastic band length along each side. Add them up. That is the perimeter. Make two shapes with the same perimeter but different shapes. Which is easier to compute perimeter for? (Regular polygons: just multiply one side by the number of sides.)

Explore

Perimeter investigation: using centimetre grid paper, draw 5 polygons with different perimeters. For each: measure all sides, calculate perimeter, record. Challenge: draw a rectangle with perimeter exactly 24 cm. How many different rectangles are possible? (1x11, 2x10, 3x9, 4x8, 5x7, 6x6.)

Consolidate

Practice

Students calculate perimeters of 6 shapes, solve 3 missing-side problems, and solve 2 real-world perimeter problems. Exit ticket: a square has perimeter 36 cm. What is the side length?

Exit ticket

Students calculate perimeters of 6 shapes, solve 3 missing-side problems, and solve 2 real-world perimeter problems. Exit ticket: a square has perimeter 36 cm. What is the side length?

TIP Always label perimeter answers with length units (cm, m). The most common error on perimeter problems is writing the number without the unit.

WORKED EXAMPLES

A regular octagon has perimeter 72 cm. What is the side length?

Regular octagon: 8 equal sides. 72/8 = 9 cm per side.

An irregular pentagon has sides 6, 8, 5, 7, and one unknown side. Perimeter = 35 cm. Find the unknown side.

6+8+5+7=26. 35-26=9 cm.

MATERIALS

•Geoboards and elastic bands

•Rulers and centimetre grid paper

•Perimeter problem cards

•Real-world context cards (fencing, framing)

WATCH FOR

!Students may add the number of sides rather than their lengths. Always ask: what are you adding? Side lengths, not side counts.

!Students may compute area instead of perimeter. Distinguish clearly: perimeter is the fence (around), area is the grass (inside).