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

Interpret conjugation translation rotation and complex division

Connect complex operations to transformations on the Argand plane, specifying their centre, direction and inverse.

Pure Mathematics 3 · Complex numbers · 3.9
Exam scope and assessment notes

Pure Mathematics 3 (Paper 3, A Level) · Mathematics 9709, 2026–2027, section 3.9. Paper 1 algebra and radian trigonometry are assumed. Use Cartesian working for products, quotients and the two square roots of a non-zero complex number. Arguments usually use −π < θ ≤ π; an alternative interval 0 ≤ θ < 2π is acceptable unless specified. The argument of zero is undefined. General nth-root methods are beyond this mapped sequence. Written practice is self-checked; human subject-teacher review remains pending.

Before you begin

  • Carry out Cartesian and polar complex operations.
  • Interpret directed translations, rotations and scale factors.

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

Quick readiness check

Which operation translates every point by (3, −1)?

What you will learn

  • Describe conjugation and addition or subtraction as geometric transformations.
  • Calculate the image and inverse of a non-zero complex multiplication.
Rotation and reflection differz = 2 + i · iz · z*ReIm0zizz*All three moduli are √5
The blue point z = 2 + i becomes the red point iz = −1 + 2i under an anticlockwise rotation of π/2 about the origin. The green point z* = 2 − i is its reflection across the real axis. The dashed circle has radius √5, the common modulus of all three points. The axes have equal scales. Multiplication by i and conjugation are distinct operations, even though both preserve this distance.

Conjugation reflects; addition translates

For z = x + iy, its conjugate x − iy reflects the point across the real axis. Points on the real axis stay fixed. Adding a fixed a = u + iv translates every point by vector (u,v), while subtracting a translates by (−u,−v). For z = 2 + i, its conjugate is 2 − i; adding 3 − i gives 5. These are different operations with different geometric effects.

A directed difference b − a represents the vector from a to b. Translation preserves distances and angles because it adds the same displacement to both endpoints. Conjugation also preserves lengths but reverses the orientation of a directed angle. Multiplication by −1 reflects through the origin, rather than across the real axis, so it is different from conjugation except at particular points.

A non-zero multiplier scales and rotates about the origin

Multiplication by q = rcis θ, r > 0, scales distances from the origin by r and rotates about the origin through θ. In particular multiplication by i is an anticlockwise rotation by π/2, because its modulus is one and its argument is π/2. The point 2 + i becomes −1 + 2i. Multiplication by 1 + i instead uses scale √2 and angle π/4, giving 1 + 3i.

Division by q is the inverse: scale 1/r and rotation −θ. To solve (1 + i)z = 4 + 2i, divide to obtain z = 3 − i. Applying the multiplier checks the inverse result. A rotation about another centre c needs a stated expression such as c + q(z − c); simply multiplying by q uses the origin. A zero multiplier collapses every point to zero and has no inverse.

Pause and explain

What does multiplication by i do?

Put the idea to work

Worked example

For z = 2 + i, calculate z*, iz and (1 + i)z and describe their transformations.

Show the worked solution
  1. Conjugation gives 2 − i, a reflection across the real axis.
  2. Multiplying by i gives −1 + 2i, a rotation by π/2 anticlockwise about the origin.
  3. Multiplying by 1 + i gives 1 + 3i, scale √2 and rotation π/4 about the origin.

Answer z* = 2 − i; iz = −1 + 2i; (1 + i)z = 1 + 3i.

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

Translate z = −1 + 2i by adding 3 − i. Give its image and the inverse translation.

Give me a hint

Add the corresponding Cartesian parts and reverse the displacement to undo it.

Compare my reasoning
  1. The image is (−1 + 3) + (2 − 1)i = 2 + i.
  2. The displacement is (3, −1).
  3. Subtracting 3 − i from 2 + i returns −1 + 2i.

Image 2 + i; inverse subtracts 3 − i.

Look for these in your work

  • Calculate the signed coordinate displacement.
  • Check the inverse operation returns the starting point.
2 · Independent

Apply multiplication by i to 3 − i. State the image, angle, centre and scale.

Give me a hint

Expand the Cartesian product and identify the multiplier’s modulus and argument.

Compare my reasoning
  1. i(3 − i) = 3i − i² = 1 + 3i.
  2. The multiplier i has modulus 1 and argument π/2.
  3. Thus the image is obtained by a π/2 anticlockwise rotation about the origin with scale 1.

Image 1 + 3i; rotation π/2 anticlockwise about the origin; scale 1.

Look for these in your work

  • Use i² = −1 in the image calculation.
  • Specify the rotation’s direction and centre.
3 · Transfer

A transformed point w = 4 + 2i was obtained by w = (1 + i)z. Find z with Cartesian working and describe the inverse geometry.

Give me a hint

Divide by 1 + i using its conjugate; the inverse scale and angle come from its polar form.

Compare my reasoning
  1. Use z = [(4 + 2i)(1 − i)]/2 = (6 − 2i)/2 = 3 − i.
  2. The multiplier has modulus √2 and argument π/4.
  3. The inverse scales by 1/√2 and rotates clockwise by π/4 about the origin; multiplication checks w.

z = 3 − i; inverse scale 1/√2 and rotation −π/4 about the origin.

Look for these in your work

  • Verify the Cartesian inverse in the supplied mapping.
  • State reciprocal scale and opposite rotation about the correct centre.

Common mistakes

  • Conjugation is a real-axis reflection, not multiplication by −1.
  • A multiplication’s rotation and enlargement use the origin unless a translated expression is supplied.
  • An inverse needs a non-zero multiplier and uses reciprocal scale and opposite rotation.
Recall without your notes

What can you explain now?

Does multiplication by i change the modulus of a point?

Compare with the explanation

No: |i| = 1.

It rotates by π/2 about the origin and preserves all distances.

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

Calculate and describe the image of a point under a complex operation, then specify whether and how the operation can be reversed.

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.