POPStudying

Cambridge 0580 IGCSE  ·  Topic E5.4

E5.4 Surface Area and Volume of Solids

Image  ·  914 × 270

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:

Curved S.A. of cylinder=2πrhCurved S.A. of cone=πrlS.A. of sphere=4πr2\text{Curved S.A. of cylinder} = 2\pi r h \qquad \text{Curved S.A. of cone} = \pi r l \qquad \text{S.A. of sphere} = 4\pi r^2
Vprism=cross-sectional area×lengthVcylinder=πr2hV_{\text{prism}} = \text{cross-sectional area} \times \text{length} \qquad V_{\text{cylinder}} = \pi r^2 h
Vpyramid=13×base area×hVcone=13πr2hVsphere=43πr3V_{\text{pyramid}} = \tfrac{1}{3} \times \text{base area} \times h \qquad V_{\text{cone}} = \tfrac{1}{3}\pi r^2 h \qquad V_{\text{sphere}} = \tfrac{4}{3}\pi r^3

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 V=area×lengthV = \text{area} \times \text{length} 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:

l=r2+h2l = \sqrt{r^2 + h^2}

This is needed constantly for curved-surface-area questions, since the curved surface area formula uses ll, not the vertical height hh.

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 13\dfrac13 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 5 cm5\text{ cm} and vertical height 12 cm12\text{ cm}. Find its total surface area in terms of π\pi.

  1. Find the slant height: l=52+122=25+144=169=13 cml = \sqrt{5^2+12^2} = \sqrt{25+144} = \sqrt{169} = 13\text{ cm}.
  2. Curved surface area: πrl=π(5)(13)=65π cm2\pi r l = \pi(5)(13) = 65\pi\text{ cm}^2.
  3. Area of the circular base: πr2=π(5)2=25π cm2\pi r^2 = \pi(5)^2 = 25\pi \text{ cm}^2.
  4. Total surface area =65π+25π=90π cm2= 65\pi + 25\pi = 90\pi \text{ cm}^2.

07 — Worked example

Worked example: volume of a pyramid needing Pythagoras first

A pyramid has a square base of side 10 cm10\text{ cm} and a slant edge (apex to a base corner) of 13 cm13\text{ cm}. Find the volume of the pyramid.

  1. The base diagonal is 102+102=200 cm\sqrt{10^2+10^2} = \sqrt{200}\text{ cm}, so half the diagonal (base centre to a corner) is 2002 cm\dfrac{\sqrt{200}}{2}\text{ cm}.
  2. The vertical height, half-diagonal, and slant edge form a right-angled triangle: h=132−(2002)2=169−50=119≈10.9 cmh = \sqrt{13^2 - \left(\tfrac{\sqrt{200}}{2}\right)^2} = \sqrt{169 - 50} = \sqrt{119} \approx 10.9\text{ cm} (3 s.f.).
  3. Volume =13×base area×h=13×100×10.9≈364 cm3= \tfrac13 \times \text{base area} \times h = \tfrac13 \times 100 \times 10.9 \approx 364\text{ cm}^3 (3 s.f.).