QCE Specialist Mathematics - Unit 1 - Combinatorics

Permutations, combinations and restrictions

Learn permutations and combinations for QCE Specialist Mathematics Unit 1 with proof-quality reasoning, KaTeX, an original diagram and checked practice.

Part of the free QCE Specialist Mathematics notes library for Unit 1: Combinatorics.

Updated 2026-08-09 - 9 min read

Distinguish ordered arrangements from unordered selections and encode restrictions without overcounting. Specialist Mathematics rewards more than obtaining a value: definitions, conditions, representation choices and the direction of each implication must remain visible. The aim is a solution another student could audit line by line without relying on the diagram being to scale or the calculator being trusted blindly.

Build the mathematical model

A permutation records order; a combination records membership only. Restrictions are handled by building valid outcomes directly, partitioning into cases, using complements, or temporarily treating a required block as one object.

Start by naming the mathematical objects and their domains. Decide whether order matters, whether a vector is free or anchored, whether a matrix acts on the left or right, whether an angle is principal or general, and whether a statement is an implication, equivalence or equation. These are structural choices. If they are wrong, technically fluent algebra can still produce an invalid conclusion.

Write the givens and target in compatible notation. A proof should identify its assumptions and conclusion; a geometric model should define points, vectors and axes; a complex-number model should state the chosen form and argument interval; a transformation model should state which action occurs first. When useful, predict the sign, quadrant, orientation, count or area scale before calculating.

Governing relationships

$ {}^nP_r=\frac{n!}{(n-r)!} $

$ {n\choose r}={}^nC_r=\frac{n!}{r!(n-r)!} $

$ n!=n(n-1)\cdots2\cdot1 $

Do not treat a formula as a license to ignore its conditions. Factorials require appropriate integer inputs, division requires a non-zero divisor, a unit vector requires non-zero magnitude, an inverse requires non-zero determinant, an argument requires a non-zero complex number, and reciprocal functions exclude denominator zeros. Record the condition beside the first use rather than attempting to repair the domain at the end.

Preserve exact values through the reasoning. Fractions, radicals, $\pi$ and exact trigonometric values expose structure that a decimal may hide. Use technology to enumerate, graph, multiply, solve or verify after the mathematical representation is established. A calculator output is evidence only when its inputs, mode, interval and interpretation are documented.

Make the concept connections

1. Preserve the structure

Ask whether swapping two selected objects creates a new outcome. If yes, order matters and a permutation is appropriate; if not, divide away the internal arrangements or use a combination.

2. Connect algebra and geometry

Repeated objects reduce distinguishable arrangements. If a multiset has $n$ objects with multiplicities $n_1,n_2,\ldots$, divide $n!$ by $n_1!n_2!\cdots$.

3. Control conditions and domains

A restriction changes the sample space. 'Together' suggests a block, 'not together' often suggests a complement, and selection from multiple groups is usually clearest as disjoint cases.

The diagram is a compressed reasoning map. Recreate it without looking, then explain what each arrow, region, axis, vector, matrix column or marked angle means. Change one assumption and predict the consequence: swap ordered and unordered selection, reverse a vector, cross a determinant through zero, move an argument across the branch cut, or reverse the order of two transformations.

Specialist topics reinforce one another. Counting arguments depend on disjoint logical cases. Vector and complex representations share plane geometry. Dot products, matrix products and complex multiplication all encode structured interactions but obey different rules. Proof habits control the use of trigonometric identities and geometric theorems. Naming the shared structure makes unfamiliar questions less fragile.

A repeatable solution method

1. State the objects, available positions and whether order changes the outcome.
2. Translate each restriction into a block, complement or disjoint case before calculating.
3. Use factorial, permutation or combination notation and justify any division for indistinguishable orderings.
4. Test the formula on a smaller set by listing all outcomes and, for probability, divide by the correctly matched total count.

This workflow should remain visible in a short-response answer. Routine algebra may be compressed, but the choice of method, a key intermediate line and the conclusion must not disappear. If a question asks for proof, examples can explore the claim but do not establish a universal result. If a question asks for a model, state observations and assumptions before solving and evaluate them after obtaining a result.

When using technology, retain an independent check. Enumerate a smaller case, expand one matrix entry manually, reconstruct a vector from components, multiply a quotient by its divisor, substitute a proposed solution, or compare a transformation's determinant with its visible area effect. Verification should test the governing structure, not repeat the same keystrokes.

Worked example

Cover the solution and reproduce it from the prompt. Beside every line, write its role: definition, representation, implication, calculation, theorem, domain check or verification. Then alter one condition and predict which lines remain valid. This separates reusable reasoning from features specific to the numbers in the example.

Write a second solution that begins from a different representation, even if the first route is shorter. Compare where the two routes use the same condition and where they make different assumptions visible. If both routes reach the same result and pass the stated verification, confidence comes from independent structure rather than repetition.

Try it yourself

Proof, technology and representation audit

Before trusting a solution, identify every non-reversible step. Squaring, cancelling, multiplying by a variable expression, taking a reciprocal, applying an inverse, selecting a principal angle and dividing into cases can lose or introduce possibilities. Record exclusions and test candidates in the original problem.

Use a second representation where possible. A counting formula can be checked by a tree for smaller values. A vector result can be checked geometrically. A complex product can be checked through modulus and argument. A matrix can be checked on basis vectors. A trigonometric identity can be checked numerically at allowed angles without confusing a numerical check with a proof.

Communication matters. Define symbols before use, keep vector and scalar notation distinct, show matrix dimensions, name circle theorems and state the domain of a proof. Finish with the requested mathematical object and any necessary condition rather than leaving an uninterpreted line of algebra.

Common mistake and repair

Repair: Use the swap test: if exchanging the order of two named members does not create a new committee, use combinations rather than permutations.

The repair should happen at the first invalid decision, not after pages of downstream algebra. Cross out only the affected line, retain valid definitions and rebuild from the last justified statement. This habit makes error analysis faster and prevents a correct-looking final value from hiding a broken argument.

Assessment transfer

For arrangements of repeated letters with two vowels required together, first form a vowel block, then correct for repeated letters inside and outside the block.

For an examination response, give the governing relation or theorem, decisive working, exact result where appropriate and an explicit conclusion. For proof, include a valid starting assumption, logically connected steps and a statement that closes the original claim. For a problem-solving and modelling task, document assumptions, observations, technology choices, verification and limitations so the solution can be read independently of the task sheet.

Unfamiliar questions often combine two familiar structures. Pause before calculating and ask which part is counting, proof, vector geometry, complex arithmetic, trigonometric structure or transformation composition. Solve those layers separately, then reconnect them and test that the final object satisfies every original condition.

Verification checklist

Check that disjoint cases cover every allowed count, compare with the unrestricted total, and verify a probability lies in $[0,1]$.

Before submitting, ask:

1. Have I stated the domain, notation and any non-zero or invertibility condition?
2. Does each implication, theorem or matrix product run in the correct direction and order?
3. Have I preserved exact structure until approximation is justified?
4. Can I verify the result using a genuinely independent representation or reverse operation?
5. Does the conclusion answer the original claim, interval, geometric object or model context?

Deliberate practice

1. Rework the example with one sign, order, direction or restriction changed and predict the result first.
2. Build a smaller case that can be enumerated or drawn completely, then compare it with the general method.
3. Write a plausible incorrect solution based on the common mistake and identify its first invalid line.
4. Translate the problem into a second representation—diagram, components, polar form, matrix action or logical statement—and reconcile the two.
5. Design an unfamiliar problem that combines this lesson with one earlier topic and state the extra verification it requires.

Syllabus coverage

• Define and use permutations.
• Use factorial notation.
• Use the notation ${}^nP_r$ to represent the number of ways of selecting $r$ objects from $n$ distinct objects where order is important.
• Solve problems that involve permutations.
• Solve problems that involve permutations with restrictions including repeated objects, specific objects grouped together and selection from multiple groups.
• Define and use combinations.
• Use the notation ${n\choose r}$ and ${}^nC_r$ to represent the number of ways of selecting $r$ objects from $n$ distinct objects where order is not important.
• Solve problems that involve combinations.
• Solve problems that involve combinations with restrictions including specific objects grouped together and selection from multiple groups.
• Model and solve problems that involve permutations and combinations including probability problems, with and without technology.

Sources

• QCAA — Specialist Mathematics 2025 v1.4 syllabus
• QCAA — Specialist Mathematics 2025 formula book
• QCAA — Specialist Mathematics syllabus and resources

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