QCE General Mathematics - Unit 1 - Similarity and scale

Similar figures, triangles and linear scale

Learn similarity and linear scale for QCE General Mathematics Unit 1 with worked reasoning, KaTeX, an original diagram and checked practice.

Part of the free QCE General Mathematics notes library for Unit 1: Similarity and scale.

Updated 2026-08-09 - 7 min read

Establish similarity before matching corresponding sides and using one consistent linear scale factor. The reliable habit is to translate the situation into labelled quantities and units before choosing a calculation. General Mathematics is practical, but “practical” does not mean informal: the model, assumptions, technology and conclusion must remain visible enough for another person to audit.

Build the mathematical model

Similar figures have equal corresponding angles and proportional corresponding lengths. The scale factor must be declared in one direction, and every side correspondence must respect the same vertex order.

Start with the question rather than the formula. Identify what must be found, which information is relevant and which conditions limit the model. Give every variable a meaning and unit. If the context contains thresholds, categories, time periods, row and column labels, geometric joins or different possible domains, mark those features before calculating. This prevents a familiar procedure from being applied to the wrong quantity.

Represent the situation in at least two ways where possible: words and an equation, a table and a graph, a labelled diagram and a formula, or a matrix and its row-column labels. Agreement between representations is useful evidence. Disagreement is a signal to stop and locate the first modelling or transcription error.

Governing relationships

$ k=\frac{\text{image length}}{\text{original length}} $

$ \frac{a'}{a}=\frac{b'}{b}=\frac{c'}{c}=k $

Write the relationship before substituting. Keep exact values or full calculator precision through intermediate steps, then round the final result to a precision justified by the supplied data and context. Currency normally requires cents, measured quantities should not imply unsupported accuracy, and counts may require a whole-number interpretation rather than ordinary decimal rounding.

Units are part of the reasoning. A rate combines two units; area uses square units; volume uses cubic units; a gradient has output units per input unit; a matrix entry inherits the labels of its row and column. If the units do not match the requested quantity, a correct-looking decimal is not a correct answer.

Make the concept connections

1. Interpret the quantities

A shared shape is not enough: the angle and side correspondence must be justified. Triangle similarity can follow from matching angles or proportional sides, but the written vertex order controls which sides correspond.

2. Connect the representations

A scale factor is directional. If an enlargement uses $k=1.5$, the reverse comparison uses $1/1.5$, not the same factor.

3. Protect the conditions

Scale drawings translate paper measurements into real measurements. Units must be converted before applying the ratio, especially when a plan uses centimetres and the real object uses metres.

The diagram above is a reasoning tool. Recreate its essential labels from memory and explain what remains fixed, what changes and how the representations connect. Then alter one condition—a threshold, scale factor, graph domain, matrix order, outlier or measurement unit—and predict which part of the diagram and method must change.

A repeatable solution method

1. Mark corresponding vertices using angle evidence or the stated drawing scale.
2. Write the direction of the scale factor and one ratio of corresponding lengths.
3. Solve the proportion without mixing sides from different correspondences.
4. Check that every image length changes by the same multiplier.

This sequence is deliberately explicit. A short-response solution can compress routine arithmetic, but it should not hide the model choice, units or contextual interpretation. Technology is valuable for repeated computations, graphs, matrix powers and statistical summaries; it does not decide whether the inputs, formula, interval, labels or conclusion are appropriate.

When using a spreadsheet or calculator, record enough working to reproduce the result: name the entered quantities, show the governing formula or command, retain the unrounded value and explain the displayed output. A screen value without interpretation is not evidence that the right question was answered.

Worked example

Cover the worked steps and reproduce the solution from the problem statement. Then change one quantity or condition and predict the direction and approximate size of the effect before recalculating. This counterfactual check distinguishes understanding from pattern copying and prepares you for unfamiliar questions.

Technology and representation audit

Technology should extend the mathematics rather than conceal it. For a spreadsheet, inspect copied references and test a corner cell. For a graph, read axes, units, scale, endpoints and domain. For a matrix, confirm dimensions and labels before entering the product. For statistics, confirm the intended variable, sample size and the correct calculator statistic. For measurement, sketch the boundary or exposed faces before using stored formulas.

Use one independent representation as a check. A graph can check an algebraic intersection; substitution can check a graph reading; a table can check a formula pattern; an estimate or bound can check measurement; a manually expanded entry can check a matrix calculation; and an ordered list or plot can check a statistical summary.

Common mistake and repair

Repair: Write the two triangles in matching vertex order and place corresponding side labels in aligned columns.

Do not repair a conceptual error by adding decimal places. Find the first decision that broke the model: a wrong denominator, unmatched time period, omitted face, reversed scale direction, invalid graph interval, incompatible matrix dimension, unsuitable summary or unsupported interpretation. Rebuild from that point and preserve the parts that were valid.

Assessment transfer

For an indirect measurement, state the physical assumptions—parallel sight lines, level ground or equal angles—that make the figures similar.

For a short-response question, show the governing relationship, substitution, result with units and one contextual sentence. For a problem-solving or modelling task, make assumptions and observations explicit, justify the chosen representation, use technology for a meaningful purpose, evaluate reasonableness and limitations, and organise the response so it can be read independently of the task sheet.

A defensible conclusion answers the question at the strength supported by the evidence. Say “for this model” or “in this sample” when generalisation is limited. State thresholds and domains. Distinguish an exact calculation from an estimate and a possible outlier from an error. These qualifications improve mathematical communication; they do not weaken it.

Verification checklist

Use a second side ratio when available and confirm the larger figure has all corresponding lengths larger when $k>1$.

Before submitting, ask:

1. Did I define the unknowns, labels, units and valid domain?
2. Does my chosen formula, graph, table, matrix or statistic match the information structure?
3. Can I reproduce the result through substitution, a second representation, a bound or a spot check?
4. Is the rounding and format appropriate for money, measurement, count or data?
5. Does the final sentence interpret the result without claiming more than the model or data support?

Deliberate practice

1. Rework the example with one input increased by 20%. Predict the direction of change first.
2. Create a plausible but incorrect solution based on the common mistake above, then annotate the exact line where it fails.
3. Represent the same situation in a second form—diagram, graph, table, spreadsheet or matrix—and explain how the two forms agree.
4. Write a one-sentence reasonableness check that uses units, bounds, a reverse operation or a contextual constraint.
5. Design an unfamiliar example in which the usual method needs one extra decision, such as a threshold, join, outlier, domain restriction or reordered category.

Syllabus coverage

• Understand the conditions for similarity of two-dimensional figures, including similar triangles.
• Use the scale factor for two similar figures to solve linear scaling problems.

Sources

• QCAA — General Mathematics 2025 v1.3 syllabus
• QCAA — General Mathematics 2025 formula book
• QCAA — General Mathematics resources

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