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A Level · Cambridge (CIE) · 9709

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

Pure Mathematics 3 · Vectors · 3.7
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.
Read 3D vectors from the verticesOABCDEFGBlue →OB = (4,3,0)ᵀRed →OF = (4,3,2)ᵀ
This is a perspective schematic of a cuboid with perpendicular edge lengths 4, 3 and 2. Its coordinates are O = (0,0,0), A = (4,0,0), B = (4,3,0), C = (0,3,0), D = (0,0,2), E = (4,0,2), F = (4,3,2) and G = (0,3,2). The blue base diagonal OB has vector (4,3,0)ᵀ and length 5. The red body diagonal OF has vector (4,3,2)ᵀ and length √29. Their actual angle has cosine 5/√29 and is approximately 21.8°. The projection distorts lengths and angles; calculate them from the 3D vectors rather than measuring the drawing. Dashed edges indicate obscured edges of the solid.

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?

Put the idea to work

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
  1. Use directions (4, 3, 0)ᵀ and (4, 3, 2)ᵀ, with scalar product 16 + 9 = 25.
  2. Their magnitudes are 5 and √29, so cos θ = 25/(5√29) = 5/√29.
  3. The requested angle is arccos(5/√29) ≈ 21.8°, using the 3D coordinates rather than the perspective projection.

Answer arccos(5/√29) ≈ 21.8°.

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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.

1 · Guided

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
  1. →AB = (−2, 3, 0)ᵀ and →AC = (−2, 0, 4)ᵀ give scalar product 4.
  2. The magnitudes are √13 and √20, so cos θ = 4/√260 = 2/√65.
  3. 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.
2 · Independent

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
  1. The directions are (0, 3, 2)ᵀ and (0, 3, 0)ᵀ, with dot product 9 and magnitudes √13 and 3.
  2. The acute angle is arccos(3/√13) ≈ 33.7°.
  3. 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.
3 · Transfer

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
  1. The foot parameter is [(4, 3, 2) ⋅ (0, 0, 2)]/4 = 1.
  2. 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.
  3. 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.
Recall without your notes

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.

After trying it yourself, choose your next review. This is your self-assessment.

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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.

Keep learning Mathematics

Course scope and assessment

312 sequenced lessons provide 936 guided, independent and application problems covering the 160 objectives in sections 1.1–1.8, 2.1–2.6, 3.1–3.9, 4.1–4.5, 5.1–5.5 and 6.1–6.5. The map lists all 38 sections across six alternative components; all eight Pure Mathematics 1 sections and all six Pure Mathematics 2 sections, from Paper 2 Algebra through Numerical solution of equations, currently have full teaching sequences. Paper 3 Algebra, section 3.1, links the six shared foundation lessons plus six lessons on partial fractions and rational binomial expansions; Paper 3 logarithms, trigonometry and numerical equations use the shared lessons, with the Paper 1 graph foundation linked into trigonometry. Paper 3 differentiation combines the shared methods with its inverse-tangent derivative lesson. Paper 3 Integration links four shared standard and identity lessons plus seven advanced lessons, covering all six section 3.5 requirements. Eight Vectors lessons cover all six section 3.7 requirements, with 2D and 3D line and solid applications. Nine Differential equations lessons cover all four section 3.8 requirements, including Paper 3 integration techniques, initial conditions, equilibrium solutions and conditional model interpretations. Eleven Complex numbers lessons cover all eight section 3.9 requirements, with seven Argand diagrams and explicit argument and boundary checks. All nine Pure Mathematics 3 sections now have teaching linked to every mapped requirement. Nine Forces and equilibrium lessons cover all seven section 4.1 requirements, with six force diagrams, signed incline-friction intervals, reaction-dependent pull and push limits and separate-body third-law checks. Nine Kinematics lessons cover the four official section 4.2 requirements, with the graph requirement mapped to three relationships. Six motion diagrams support directed quantities, graph slopes and signed areas, Paper 1 time-calculus methods, reversal distances, constant-acceleration stages and shared-clock particle meetings. Six Momentum lessons cover both section 4.3 requirements, including signed system totals, conservation assumptions, outgoing velocities, coalescence, unknown masses and successive impacts. Four diagrams show system momentum through the stipulated events, and supplied kinetic-energy checks distinguish conservation of momentum from changes in kinetic energy. Twelve Newton’s laws lessons cover all four section 4.4 requirements, with vertical support and release stages, rough horizontal and incline motion, inferred parameters, hanging and surface pulley systems, rigid tow-bar thrust and slack-string transitions. Seven force diagrams support the separate-particle balances. Nine Energy, work and power lessons cover all five section 4.5 requirements, with angle-dependent work, kinetic and reference potential energy, signed net-work balances, smooth curved slides, rough and driven slopes, ideal connected-system energy and instantaneous power on hills. Five diagrams distinguish the energy inputs, losses and valid positive-speed force model. All five Mechanics sections now have teaching and staged practice linked to their mapped requirements. Thirteen Representation of data lessons cover all five section 5.1 requirements, including choice and limitations of displays, ordered and back-to-back stems, raw summaries and box plots, unequal-width density histograms, cumulative-frequency interpolation, descriptive moment calculations, grouped estimates, coding, combining two datasets and correcting totals. Six accessible diagrams retain repeated observations and distinguish numerical spread, frequency area and estimated readings. Ten Permutations and combinations lessons cover both official section 5.2 requirements, with selection and ordered roles, repeated objects, adjacency blocks, non-adjacency gaps, leading-zero cases, labelled multi-row seating and mixed restrictions. Six accessible diagrams show outcome trees, ordered versus unordered choices, repeated copies, blocks, gaps and distinct row seats. Eleven Probability lessons cover all four section 5.3 requirements, with equiprobable elementary outcomes, selection-based probabilities, complements and disjoint regions, independence tests, restricted sample spaces, two-way tables, replacement and removal trees, reversed conditions and unknown parameters. Seven accessible diagrams show dice outcomes, random subsets, event regions, a conditioned grid, a count table and two probability trees. Twelve Discrete random variables lessons cover all three official section 5.4 requirements, with complete probability tables, moment and parameter checks, binomial assumptions, exact probabilities and moments, geometric first-success probabilities, tails, expectation and integer reliability thresholds. Six accessible diagrams distinguish outcome grouping, weighted moments, fixed trials, discrete probability bars, waiting conventions and integer tail deadlines. Twelve Normal distribution lessons cover all three official section 5.5 requirements, with continuous-area interpretation, MF19 cumulative tables, full standardisation, tail and interval regions, conditional probabilities, percentiles, one- and two-parameter inference, binomial suitability, continuity corrections and approximate integer thresholds. Seven accessible diagrams show model areas, table reading, aligned scales, intervals, quantiles, matched binomial and normal shapes and whole integer cells. All five Statistics 1 sections now have teaching and staged practice linked to their mapped requirements. Fourteen Poisson distribution lessons cover all five official section 6.1 requirements, with homogeneous event-count assumptions, exposure scaling, exact masses and tails, conditional count events, moments, parameter inference, probability duration targets, disjoint and overlapping windows, rare-count binomial approximations, matched normal approximations and continuity corrections. Nine accessible diagrams show event exposures, unit conversions, discrete bars, complete tail complements, population moments, shared windows, approximate count models and integer cells. Exact checks distinguish approximate and exact close-target decisions. Fourteen Linear combinations lessons cover the six listed results in official section 6.2, with affine moments, signed scales and constant cases, expectation without independence, independent squared variance contributions, fixed and random fees, independent copies versus reused values, affine normal events, independent normal sums and differences, weighted and conditional areas, coefficient inference, exact independent Poisson sums and overlapping exposure limits. Nine accessible diagrams distinguish scale changes, variance contributions, component costs, copy mechanisms, aligned normal scales, summed areas, Poisson count pairs and unsupported distribution claims. Fourteen Continuous random variables lessons cover both official section 6.3 requirements, with density validity and units, support-aware intervals, complements and conditional regions, first and second moments, percentiles by direct density areas, symmetry checks, parameter inference, infinite-support power and exponential integrals, divergent-moment limits and combined applications. Nine accessible diagrams distinguish density height from probability, normalisation, negative-density failures, central and conditional areas, mean versus median, percentile locations and partial infinite tails. Explicit cumulative distribution function knowledge is not required. Eighteen Sampling and estimation lessons cover all eight official section 6.4 requirements, with population and frame definitions, random-number selection, sampling-quality limits, independent sample-mean moments, normal means, CLT conditions, SE scaling, raw, summarised and coded unbiased estimates, known-variance and large-sample mean intervals, repeated-sampling interpretation, precision planning, proportion intervals, sparse-outcome limits and contextual applications. Ten accessible diagrams distinguish selection frames, number rejection, repeated sample means, population versus mean density, CLT models, variance divisors, interval targets, illustrative coverage, integer size checks and proportion margins. Eighteen Hypothesis tests lessons cover all five official section 6.5 requirements, with pre-specified alternatives, inclusive count tails, explicit equal-tail conventions, integer critical regions and actual significance, exposure-aware Poisson tests, continuity-corrected count approximations, normal and large-sample mean tests, confidence-interval connections, error classifications, fixed-region binomial, Poisson and normal error probabilities and sample-size power comparisons. Nine accessible diagrams show the testing procedure, exact count regions, unequal Poisson tails, discrete significance jumps, corrected count boundaries, matching mean intervals and alternative error distributions. All shared lessons are counted once in the course total. All 38 sections now link teaching and practice. Eight earlier overview lessons remain available, but do not count as full section coverage. This is a content-availability measure, not full programme completion or exam readiness. Written practice is self-checked, not officially marked. Check the 2028–2030 syllabus separately if sitting an examination from 2028; this teaching map uses 2026–2027 requirements.

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