Cut and Slide

Parallelograms, triangles, trapezoids

An interactive lesson with 7 steps, about 6 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Parallelogram: $A = bh$, using the perpendicular height, not the slanted side.
• Triangle: $A = \tfrac12 bh$. Trapezoid: $A = \tfrac12(a + b)h$.
• Cutting and rearranging doesn’t change area. It’s the key to these formulas.

Steps

1. A leaning rectangle
2. Slice and slide
3. Triangles are half
4. Triangle area
5. Slide the top
6. Trapezoid
7. On the grid