Every flat shape holds two different questions: how much space is INSIDE (the area, measured in square units like $\mathrm{m^2}$ — metres squared), and how far it is AROUND (the perimeter, in plain metres). A rug is area; its fringe is perimeter. Keep those two apart and half this chapter's traps never touch you.

Four area machines to meet, each with a read-aloud form you can own. In every one $A$ is the area, and every letter beside it is a length in metres. $A = l \cdot w$ for rectangles (A equals l times w), $l$ the length, $w$ the width. $A = \tfrac{1}{2} b h$ for triangles (A equals one-half b h), $b$ the base, $h$ the height standing straight up from it. $A = \pi r^2$ for circles (A equals pi r squared: $r$ is the radius, centre to edge, and $\pi$ is the Greek letter pi, said “pie”, a little more than 3). And $A = \dfrac{a+b}{2} \cdot h$ for trapezoids (A equals a plus b over two, times h): $a$ and $b$ the two parallel sides, $h$ the distance between them. Today you only match shape to machine — the computing starts next lesson.