Area & Perimeter
Rectangles, triangles, and circles from their basic measurements.
Concept
Area counts squares, perimeter walks the edge
Rectangle: area = length × width, perimeter = 2(length + width).
Triangle: area = ½ × base × height, where the height is perpendicular to that base — not a slanted side.
Circle: area = πr², circumference = 2πr.
A▭ = lw · A△ = ½bh · A○ = πr²