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A Level · Cambridge (CIE) · 9709

Calculate Cartesian products quotients and conjugates

Add and subtract components, expand products with i² = −1 and divide by multiplying by the denominator’s conjugate.

Section 3.9 · Complex numbers

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

  • Collect signed real and imaginary terms.
  • Expand binomials and simplify fractions.

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

Quick readiness check

What is the conjugate of 2 − 3i?

What you will learn

  • Carry out Cartesian addition, subtraction and multiplication with full working.
  • Use a conjugate to divide by a non-zero complex number.

Addition and multiplication use different rules

For z = x + iy and w = a + ib, add or subtract corresponding parts: z + w = (x + a) + i(y + b). For multiplication expand every term and use i² = −1: zw = (xa − yb) + i(xb + ya). With z = 3 + 2i and w = 1 − i, their sum is 4 + i, difference is 2 + 3i and product is 5 − i. Multiplying imaginary coefficients without the negative i² contribution changes the real part incorrectly.

The conjugate z* = x − iy reflects the sign of the imaginary part. The product zz* = x² + y² is real and non-negative, and vanishes only at z = 0. For 3 + 2i the conjugate is 3 − 2i and their product is 13. Conjugation preserves a real number and reverses a purely imaginary number; it does not mean changing the signs of both parts.

A conjugate makes a complex denominator real

To calculate z/w, w ≠ 0, multiply numerator and denominator by w*. The denominator becomes a² + b², so the quotient is [(xa + yb) + i(ya − xb)]/(a² + b²). Show the expansion and simplification when Cartesian working is required. For (3 + 2i)/(1 − i), multiply by 1 + i to obtain (1 + 5i)/2.

A complex equation such as (1 + 2i)z = 3 − i can be solved by this same division because the multiplier is non-zero. Then check the proposed quotient by multiplying it by the original denominator. Dividing by zero is undefined in the complex system just as it is in real arithmetic; the conjugate method cannot make a zero denominator valid.

Pause and explain

Which factor makes the denominator 2 − i real?

Put the idea to work

Worked example

Calculate (3 + 2i)/(1 − i), showing Cartesian working.

Show the worked solution
  1. Multiply top and bottom by 1 + i.
  2. Expand the numerator: 3 + 3i + 2i + 2i² = 1 + 5i; the denominator is 1 − i² = 2.
  3. Thus the quotient is (1 + 5i)/2; multiplication by 1 − i returns 3 + 2i.

Answer (1 + 5i)/2.

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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

Calculate (2 − 3i)(−1 + 4i), showing the expansion.

Give me a hint

The product of the two imaginary terms uses i² = −1.

Compare my reasoning
  1. Expand to −2 + 8i + 3i − 12i².
  2. Replace i² by −1, so the real terms total 10.
  3. Collect the imaginary terms to obtain 10 + 11i.

10 + 11i.

Look for these in your work

  • Show all four terms in the Cartesian product.
  • Use the correct sign for the i² term.
2 · Independent

Calculate (4 + i)/(2 − i) in Cartesian form with full working.

Give me a hint

Multiply both parts of the fraction by the conjugate 2 + i.

Compare my reasoning
  1. Use [(4 + i)(2 + i)]/[(2 − i)(2 + i)].
  2. The numerator is 8 + 6i + i² = 7 + 6i, and the denominator is 5.
  3. Hence z = (7 + 6i)/5; multiplying by 2 − i recovers 4 + i.

(7 + 6i)/5.

Look for these in your work

  • Use the denominator’s conjugate on top and bottom.
  • Simplify and verify the original quotient.
3 · Transfer

Solve (1 + 2i)z = 3 − i in Cartesian form and verify your answer.

Give me a hint

Divide by the non-zero multiplier using its conjugate.

Compare my reasoning
  1. Multiply the quotient (3 − i)/(1 + 2i) by (1 − 2i)/(1 − 2i).
  2. The numerator is 1 − 7i and the denominator is 5, giving z = (1 − 7i)/5.
  3. Multiplication by 1 + 2i gives (15 − 5i)/5 = 3 − i.

z = (1 − 7i)/5.

Look for these in your work

  • Solve by a valid non-zero division.
  • Verify the result in the original complex equation.

Common mistakes

  • Expand all four product terms and replace i² by −1.
  • Conjugation changes only the imaginary sign.
  • Multiply numerator and denominator by the same conjugate and exclude a zero denominator.
Recall without your notes

What can you explain now?

What is (2 + i)(2 − i)?

Compare with the explanation

5.

The middle terms cancel and −i² contributes +1 to the real product.

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

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Make it stick

Expand a Cartesian product, form a conjugate and divide with a real denominator, showing each algebraic step.

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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