Today's lesson · 4 August 2026
Area of triangles, parallelograms and composite shapes
Deriving the triangle and parallelogram area formulas from a rectangle, then combining them to find the area of L-shaped and composite figures
Today's lesson is Area of triangles, parallelograms and composite shapes, a Year 7-8 (ages 12 to 14) Math unit. Today's featured teaching unit. No textbook needed, just open it and teach.
How do you find the area of a shape that is not a rectangle?
Every area formula you will ever use for a straight-sided shape traces back to one idea: the area of a rectangle, length times width. A triangle is exactly half of a rectangle that encloses it. A parallelogram can be cut and shifted into a rectangle of the exact same base and height. And an L-shaped room, an oddly shaped block of land, or a floor plan with a bay window, a 'composite' shape, is just several rectangles (or rectangle-minus-rectangle) added or subtracted together.
What students will be able to do
Students will find the area of a triangle using A = 1/2 x base x height, find the area of a parallelogram using A = base x height, and find the area (and perimeter) of a composite shape by adding or subtracting the areas of simpler rectangles that make it up.
Curriculum anchor
Teach the whole lesson
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