LESSON PLAN

Area of Triangles, Parallelograms, and Trapezoids

Apothem Team

Grade 6 · Measurement

LESSON AT A GLANCE

Warm-up

5 min

Explore

15 min

Formalize

10 min

Practice

12 min

Exit ticket

3 min

Warm-up

Review area of a rectangle. What if we cut off a corner?

Explore

Students cut rectangles and rearrange pieces to form triangles, parallelograms, and trapezoids. They measure and discover the patterns. Why is a triangle half? Why is a parallelogram the same area as a rectangle?

Formalize

Write the three formulas side by side:

Triangle: A = (1/2)bh | Parallelogram: A = bh | Trapezoid: A = (1/2)(b₁ + b₂)h

Connect each formula back to rectangles. Students should see why, not just memorize.

Practice

Students find area of triangles, parallelograms, and trapezoids on grid paper and with given measurements. Exit ticket: one of each type.

Exit ticket

Students find area of triangles, parallelograms, and trapezoids on grid paper and with given measurements. Exit ticket: one of each type.

TIP Hands-on manipulation is key. Let students cut, fold, and rearrange before introducing formulas.

WORKED EXAMPLES

A triangle has base 6 cm and height 4 cm. What is its area?

A = (1/2) × 6 × 4 = 12 cm².

A trapezoid has bases 5 cm and 7 cm, and height 3 cm. What is its area?

A = (1/2) × (5 + 7) × 3 = (1/2) × 12 × 3 = 18 cm².

MATERIALS

•Grid paper

•Scissors

•Rulers

•Geoboards

•Rectangular paper cutouts

WATCH FOR

!Using a slant side as the height instead of the perpendicular distance.

!Forgetting the (1/2) factor for triangles and trapezoids.

!Confusing base with any side of the shape.