Solve vector geometry inside cuboids and tetrahedra
Build coordinates from perpendicular edges, form directions from the correct endpoints and calculate 3D angles or perpendicular distances.
Section 3.7 · Vectors
Exam scope and assessment notes
Pure Mathematics 3 (Paper 3, A Level) · Mathematics 9709, 2026–2027, section 3.7. Paper 1 coordinate geometry and algebra are assumed. Work in two or three dimensions as stated. The general section theorem is not included; midpoint geometry is required. The cross product, shortest distance between skew lines and their common perpendicular are not required. Scalar products include angles between lines and the foot of a perpendicular from a point to a line, including 3D solids. Written practice is self-checked; human subject-teacher review remains pending.
Before you begin
- Use displacement, magnitudes, scalar products and vector line equations.
- Find a point-to-line perpendicular foot and check a segment's bounds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
For angle BAC, which directed vectors share the correct starting vertex?
What you will learn
- Assign a consistent 3D frame to a solid and calculate its edge, face-diagonal and body-diagonal vectors.
- Use scalar products for angles at a specified vertex, between lines, or for a perpendicular foot on an edge.
Choose a frame and name the endpoints before calculating
For a rectangular cuboid with perpendicular edges 4, 3 and 2, choose O = (0, 0, 0), A = (4, 0, 0), C = (0, 3, 0) and D = (0, 0, 2). Completing the vertices gives B = (4, 3, 0), E = (4, 0, 2), G = (0, 3, 2) and F = (4, 3, 2). The base diagonal →OB is (4, 3, 0)ᵀ with length 5, while the body diagonal →OF is (4, 3, 2)ᵀ with length √29. A different consistent frame is valid; do not mix coordinates from different conventions.
The angle between OB and OF at O has cosine 25/(5√29) = 5/√29, so it is approximately 21.8°. The perspective diagram identifies vertices and directions but distorts lengths and angles. Calculate from the 3D components rather than measuring an angle in the drawing. At another vertex, subtract that vertex from both finishing positions to obtain the two directions of the requested angle.
The same methods work on tetrahedra and skew solid lines
For tetrahedron OABC with O = (0, 0, 0), A = (2, 0, 0), B = (0, 3, 0) and C = (0, 0, 4), the angle BAC uses →AB = (−2, 3, 0)ᵀ and →AC = (−2, 0, 4)ᵀ. Their dot product is 4 and magnitudes are √13 and √20. The cosine is 2/√65, giving approximately 75.6°. Using →OB and →OC instead would calculate the right angle at O, not the requested angle at A.
In the cuboid, lines AF and OC have directions (0, 3, 2)ᵀ and (0, 3, 0)ᵀ. Their acute angle has cosine 3/√13 and is approximately 33.7°, even though these lines are skew. Their angle is defined by directions; finding it does not require a shortest distance between skew lines. For a point-to-edge foot, use the usual zero scalar product and then confirm the resulting point is within the finite edge.
Pause and explain
Can the angle in a perspective drawing be used as the exact angle between two 3D diagonals?
Worked example
For the cuboid with O = (0, 0, 0), B = (4, 3, 0) and F = (4, 3, 2), find the angle between OB and OF at O.
Show the worked solution
- Use directions (4, 3, 0)ᵀ and (4, 3, 2)ᵀ, with scalar product 16 + 9 = 25.
- Their magnitudes are 5 and √29, so cos θ = 25/(5√29) = 5/√29.
- The requested angle is arccos(5/√29) ≈ 21.8°, using the 3D coordinates rather than the perspective projection.
Answer arccos(5/√29) ≈ 21.8°.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For tetrahedron OABC with O = (0, 0, 0), A = (2, 0, 0), B = (0, 3, 0) and C = (0, 0, 4), find angle BAC to one decimal place.
Give me a hint
Subtract A from B and C, then use the angle formula for those directions.
Compare my reasoning
- →AB = (−2, 3, 0)ᵀ and →AC = (−2, 0, 4)ᵀ give scalar product 4.
- The magnitudes are √13 and √20, so cos θ = 4/√260 = 2/√65.
- Angle BAC is arccos(2/√65) ≈ 75.6°; the vectors from O would instead describe another vertex.
Angle BAC = arccos(2/√65) ≈ 75.6°.
Look for these in your work
- Form both displacements from the named vertex.
- Use the actual 3D magnitudes and final-only rounding.
In the stated 4 × 3 × 2 cuboid, find the acute angle between lines AF and OC, and explain whether they must intersect for this angle to be defined.
Give me a hint
Use F − A and C − O as directions; compare their fixed x coordinates if checking intersection.
Compare my reasoning
- The directions are (0, 3, 2)ᵀ and (0, 3, 0)ᵀ, with dot product 9 and magnitudes √13 and 3.
- The acute angle is arccos(3/√13) ≈ 33.7°.
- AF has constant x = 4 and OC has constant x = 0, so they cannot intersect; their non-parallel directions make them skew. The angle still uses those directions.
Acute angle arccos(3/√13) ≈ 33.7°; the lines are skew and need not intersect for their angle to be defined.
Look for these in your work
- Use between-endpoint direction vectors.
- Distinguish an angle between directions from a claim of intersection.
For the cuboid, find the foot from F = (4, 3, 2) to the edge OD from O = (0, 0, 0) to D = (0, 0, 2), and the distance from F to that finite edge.
Give me a hint
Parametrise OD as t(0, 0, 2)ᵀ for 0 ≤ t ≤ 1, then impose perpendicularity.
Compare my reasoning
- The foot parameter is [(4, 3, 2) ⋅ (0, 0, 2)]/4 = 1.
- The foot is D = (0, 0, 2), within the edge's closed bounds; D→F = (4, 3, 0)ᵀ has zero scalar product with its direction.
- The distance is √(16 + 9) = 5; because the full-line foot belongs to the segment, it is also the nearest edge point.
Foot D = (0, 0, 2); distance to finite edge OD is 5.
Look for these in your work
- Verify perpendicularity and edge membership.
- Use the actual 3D separation rather than a projected drawing length.
Common mistakes
- Choose both directions from the vertex named in the angle.
- A perspective projection does not preserve the solid's actual angles or lengths.
- A skew-line angle can be calculated from directions without introducing an excluded shortest-distance method.
What can you explain now?
In the stated cuboid, what is the magnitude of →OF = (4, 3, 2)ᵀ?
Compare with the explanation
√29.
The three perpendicular component squares sum to 16 + 9 + 4 = 29.
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Make it stick
Assign coordinates to a solid, form directions from the correct endpoints and calculate an angle or foot. Explain why a perspective picture cannot supply the exact geometric measurement.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.