2D Plane Figures

Area and perimeter of common shapes and circles, plus the Pythagorean theorem.

This page is a free outline of what the 2D Plane Figures lesson covers — it is not the lesson itself. The interactive lesson, practice, quizzes, and AI Tutor are inside the app.

This lesson covers

• Circle – Area →
• Circle – Circumference →
• Circle – Parts and Properties
• Circle – Arc Length
• Circle – Sector Area
• Composite Shapes (2D) ⓘA composite shape is made of simpler shapes joined together; find its area by breaking it into familiar pieces (rectangles, triangles, circles) and adding — or subtracting a cutout.
• Scale Factor – Area
• Parallelogram – Area ⓘThe area of a parallelogram is base × height, where the height is the perpendicular distance between the bases — not the slanted side.
• Parallelogram – Perimeter ⓘThe perimeter of a parallelogram is the sum of all four sides, or 2 × (side + base) since opposite sides are equal.
• Properties of 2-D Figures ⓘFlat shapes are told apart by their properties — number of sides, equal or parallel sides, and angle measures — so a square, rectangle, and parallelogram each have a distinct set.
• Pythagorean Theorem →
• Rectangle – Area ⓘThe area of a rectangle is length × width — the flat space inside it. A 5-by-3 rectangle has area 15 square units.
• Rectangle – Perimeter ⓘThe perimeter of a rectangle is the distance around it: 2 × (length + width), since opposite sides are equal.
• Trapezoid – Area ⓘThe area of a trapezoid is ½(b₁ + b₂) × h — it averages the two parallel bases, then multiplies by the height between them.
• Triangle – Area →
• Triangle – Perimeter ⓘThe perimeter of a triangle is simply the sum of its three side lengths — the total distance around the outside.

Linked concepts (purple →) have a full explanation. Hover or tap a dotted ⓘ concept for a quick definition.