Geometry & measures · Perimeter, area & volume
Chapter 1 · 4
The idea
Surface area of prisms, cylinders, cones & spheres
Surface area as the area of the unfolded net — counting a prism's faces without missing any, why a cylinder's curved surface is 2πrh, cones and the slant height, spheres, and the hemisphere trap.
A full journey — read it, play with it, work it, then earn real exam marks. Everything stays on the timeline below.
In this lesson — start anywhere
Geometry & measures · Perimeter, area & volume
Surface area of prisms, cylinders, cones & spheres
Surface area as the area of the unfolded net — counting a prism's faces without missing any, why a cylinder's curved surface is 2πrh, cones and the slant height, spheres, and the hemisphere trap.
Why it works
The area of the net
Surface area is the area of the net. Imagine unfolding the solid flat and finding the area of every face, then adding. That single picture prevents the two standard errors: missing a hidden face, and mixing surface area (cm²) up with volume (cm³). Every solid on this page is just a different net.Prisms: two ends plus a wrap
A prism's surface is its two identical cross-sections plus a rectangle for each edge of the cross-section, all of length equal to the prism's length. For a triangular prism that is triangles rectangles — count them off on the net so none escape, because the missing-face error lives here.Keep reading — free
The rest of the explanation, plus 2 worked examples you step through move by move.
Start freeTakes a minute — no card.