01 — Explanation
Which formulas are given to me, and which must I know by heart?
Cambridge's List of Formulas gives you the curved surface area of a cylinder and cone, the surface area of a sphere, and the volume of a prism, pyramid, cylinder, cone and sphere:
You are not given the formula for total surface area of a cuboid or the flat circular ends of a cylinder/cone — you build these from area facts you already know (e.g. total surface area of a cylinder = curved surface area + area of the two circular ends).
02 — Explanation
What does "prism" actually mean here, and why does it matter?
The syllabus defines a prism as any solid with a uniform (identical, unchanging) cross-section running through its length — this includes shapes like a triangular prism, but also less obvious cases such as a cylindrical sector (a "wedge" cut from a cylinder). Recognising the constant cross-section is what tells you to use rather than a cone/pyramid-style formula.
03 — Explanation
How do I find the slant height l of a cone if it isn't given directly?
The slant height, the radius, and the vertical height of a cone form a right-angled triangle, so:
This is needed constantly for curved-surface-area questions, since the curved surface area formula uses , not the vertical height .
04 — Explanation
How do I find the height (not the slant height) of a pyramid when I'm only given its slant edge or slant face height?
Similar reasoning applies to pyramids: the vertical height, the distance from the centre of the base to the midpoint of a side (for the slant height of a triangular face), or the distance to a base vertex (for a slant edge), together form right-angled triangles you can solve with Pythagoras. Sketching the specific right-angled triangle hidden inside the 3D solid — rather than trying to picture the whole pyramid at once — makes it far easier to see which two lengths you already know and which one you're solving for.
05 — Explanation
How do I know whether to use \dfrac13 in a volume formula?
The factor appears only for pyramids and cones — solids that taper to a single point (an apex). A prism or cylinder has the same cross-sectional area all the way along its length, so no such factor is needed; a pyramid or cone's cross-section shrinks steadily to zero at the apex, and it's a geometric fact (provable using calculus, though you don't need to prove it here) that this tapering means the volume is exactly one third of the equivalent prism/cylinder with the same base and height.
Exam tip
Double-check whether a cone question gives you the vertical height or the slant height — using vertical height directly in the curved surface area formula (instead of calculating slant height first via Pythagoras) is a very common error.
06 — Worked example
Worked example
A cone has base radius and vertical height . Find its total surface area in terms of .
- Find the slant height: .
- Curved surface area: .
- Area of the circular base: .
- Total surface area .
07 — Worked example
Worked example: volume of a pyramid needing Pythagoras first
A pyramid has a square base of side and a slant edge (apex to a base corner) of . Find the volume of the pyramid.
- The base diagonal is , so half the diagonal (base centre to a corner) is .
- The vertical height, half-diagonal, and slant edge form a right-angled triangle: (3 s.f.).
- Volume (3 s.f.).