Three-dimensional geometry: distance, solids and angles

Maths: Applications and Interpretation · Geometry and trigonometry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

In three dimensions a point has three coordinates (x, y, z). The distance between two points is found with Pythagoras used on all three differences, and the midpoint is the average of each coordinate.

For solids you need the volume and surface area formulas for pyramids, cones, spheres and hemispheres. A combined solid is split into simple parts: add the volumes, but for surface area count only the faces that are on the outside.

To find an angle in a solid, draw a right-angled triangle inside it. The angle between a line and a plane is the angle between the line and its projection (its shadow) on that plane.

Rules to remember

Common mistake

Students use the slant height in the volume formula of a cone or pyramid. Volume needs the vertical height h; the slant height l is only for the curved surface area.

Worked example

A right pyramid has a square base of side 10 cm and a vertical height of 12 cm. Find the angle between a sloping edge and the base.

  1. The projection of the edge on the base runs from a corner to the centre: half the diagonal = (10√2) ÷ 2 = 7.071 cm.
  2. The right-angled triangle has opposite side 12 cm (height) and adjacent side 7.071 cm.
  3. tan θ = 12 ÷ 7.071 = 1.697, so θ = tan−1(1.697) = 59.5°.

Answer: 59.5° (to 3 significant figures)

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