Learn in your language
Skip to content
A Level · Cambridge (CIE) · 9709

Add vectors and use midpoint and parallelogram geometry

Interpret component arithmetic as directed paths, use scalar multipliers, and find midpoints relative to a common origin.

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

  • Distinguish positions and finish-minus-start displacements.
  • Add signed components and interpret a scalar multiplier.

Work on paper. Try each question before revealing help, and explain why your method works.

Quick readiness check

What is −2(3, −1)ᵀ?

What you will learn

  • Add, subtract and multiply vectors by scalars with a geometric interpretation.
  • Use the parallelogram rule and the midpoint position (a + b)/2.
Add adjacent edges to reach B→OA + →OC = →OBOABCMBoth diagonals meet at M = (2, 2)
OABC is a parallelogram in that order, with O = (0, 0), A = (3, 1), C = (1, 3) and B = (4, 4). The adjacent displacements OA and OC add to diagonal OB. The red diagonal and dashed diagonal AC meet at M = (2, 2), the midpoint of each diagonal. The horizontal and vertical scales are equal. The side midpoint of AB is a different point, (3.5, 2.5), not M.

Vector addition joins directed paths

Place the tail of a second displacement at the head of the first. Their sum is the displacement from the original start to the final endpoint. Component arithmetic records the same operation: (3, 1)ᵀ + (1, 3)ᵀ = (4, 4)ᵀ. A scalar multiplies every component. Positive scalars keep direction, negative scalars reverse it, and zero gives the zero vector; the magnitude is multiplied by the scalar's absolute value.

For parallelogram OABC, with vertices in that order and O the origin, →OB = →OA + →OC. Taking A = (3, 1) and C = (1, 3) gives B = (4, 4). The two routes O→A→B and O→C→B produce the same diagonal displacement. More generally, for parallelogram PQRS in that order, r = q + s − p; subtracting the non-origin starting position is essential.

A midpoint averages positions, not magnitudes

If A and B have positions a and b, the midpoint M has position m = (a + b)/2. Then →AM = m − a = (b − a)/2 and →MB = b − m = (b − a)/2, verifying both equal directed halves. The average applies component by component in two or three dimensions. It is not the average of the position-vector lengths, and it does not depend on either point being the origin.

In the plotted parallelogram, the midpoint of diagonal OB is (2, 2). The midpoint of the other diagonal AC is also (2, 2), so the diagonals bisect one another. The midpoint of side AB is instead (3.5, 2.5); check which pair of endpoints the question names. The syllabus requires this midpoint result, while the general section theorem for arbitrary division ratios is not included.

Pause and explain

For parallelogram OABC with O the origin, which vector reaches the opposite vertex B?

Put the idea to work

Worked example

OABC is a parallelogram with O = (0, 0), A = (3, 1) and C = (1, 3). Find B and the common midpoint of its diagonals.

Show the worked solution
  1. Use →OB = →OA + →OC to obtain B = (4, 4).
  2. The midpoint of OB is (2, 2).
  3. The midpoint of AC is [(3, 1) + (1, 3)]/2 = (2, 2), confirming that both diagonals share it.

Answer B = (4, 4); common diagonal midpoint (2, 2).

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.

1 · Guided

Find the midpoint of A = (2, −1, 4) and B = (−2, 5, 2), then verify the two half-displacements.

Give me a hint

Average each coordinate and use finish minus start for both halves.

Compare my reasoning
  1. The midpoint is M = (0, 2, 3).
  2. →AM = M − A = (−2, 3, −1)ᵀ.
  3. →MB = B − M = (−2, 3, −1)ᵀ, equal to the first half-displacement.

M = (0, 2, 3); →AM = →MB = (−2, 3, −1)ᵀ.

Look for these in your work

  • Average the signed endpoint coordinates.
  • Verify the two directed halves rather than their lengths alone.
2 · Independent

PQRS is a parallelogram in that order. P = (1, 2, −1), Q = (4, 1, 1) and S = (−1, 5, 2). Find R and check the opposite-edge displacements.

Give me a hint

Use r = q + s − p because P is not the origin.

Compare my reasoning
  1. R = (4, 1, 1) + (−1, 5, 2) − (1, 2, −1) = (2, 4, 4).
  2. →PQ = (3, −1, 2)ᵀ equals →SR = R − S.
  3. →PS = (−2, 3, 3)ᵀ equals →QR = R − Q, verifying the vertex order.

R = (2, 4, 4); →PQ = →SR = (3, −1, 2)ᵀ; →PS = →QR = (−2, 3, 3)ᵀ.

Look for these in your work

  • Subtract the non-origin starting position.
  • Check both pairs of opposite directed edges.
3 · Transfer

A supplied 2D motion model adds velocity vectors u = (3, 1) m/s and v = (−1, 2) m/s. Starting at (5, −2) m, find its constant resultant velocity, displacement after 4 s and final position.

Give me a hint

Add velocities before multiplying by the elapsed time.

Compare my reasoning
  1. The resultant velocity is (2, 3) m/s.
  2. In 4 s the displacement is 4(2, 3) = (8, 12) metres.
  3. The final position is (5, −2) + (8, 12) = (13, 10) metres.

Velocity (2, 3) m/s; displacement (8, 12) m; final position (13, 10) m.

Look for these in your work

  • Add quantities with matching vector units before scaling.
  • Distinguish velocity, displacement and final position.

Common mistakes

  • A parallelogram sum assumes the stated vertex order; identify the opposite vertex correctly.
  • Average the endpoint positions rather than the vector magnitudes.
  • A negative scalar reverses direction; it does not give a negative magnitude.
Recall without your notes

What can you explain now?

What is the midpoint position between positions a and b?

Compare with the explanation

(a + b)/2.

Its displacement from a and the displacement from it to b both equal (b − a)/2.

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

Your review choice appears on Today. Sign in to sync it across devices.

Make it stick

Explain vector addition as a path, find a non-origin parallelogram vertex, and verify a midpoint by the two equal directed half-displacements.

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.

StudyForA · Learn with understanding

Original teaching content and links to official sources. Check your exam year and qualification before using a resource.