Recognise similar figures and match corresponding parts
Match corresponding parts, test one common length ratio and explain why orientation does not decide similarity.
Exam scope and assessment notes
The first four lessons are shared Foundation content inherited by Higher; the last three are additional Higher area, volume and mixed-similarity work. Match corresponding vertices and keep one positive length factor in a stated direction. Maps and plans use exact stated lengths; screen centimetres, physical ruler accuracy and print calibration are not certified. Similar solids require corresponding dimensions to scale together; total surface area includes the whole stated external boundary. Exact numerical comparisons and text-entry attempts do not approve written proof, marks, mastery or physical construction. Linear 4MA1 Issue 2 is checked against November 2026/June 2027 entries; regional options depend on centre eligibility and modular awards remain separate. Publisher links are navigation only: literal Book2 Shape and Space6 F4.11B, Shape and Space7 F4.11A, and Shape and Space9 H4.11A/B/C tags are retained, with no Book1 tag invented. The map/proof page and topic-location conflicts remain explicit; the relevant chapter/Hub bodies are unavailable. Full-book, specialist and formal-assessment approval remain pending.
Before you begin
- Compare ratios and use a common unit for two lengths.
- Recognise triangle and quadrilateral vertices, sides and angle notation.
- Distinguish a concave reflex angle from the smaller gap between its arms.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which statement correctly compares the ratios 2:5 and 6:15?
What you will learn
- Match corresponding vertices, sides and angles in figures shown in different orientations.
- Test all relevant length ratios and retain corresponding angle magnitudes.
- Distinguish similarity from congruence and from equal-angle information alone.
Triangle ABC has sides AB=3, BC=4, CA=5 cm. Triangle DEF has DE=6, EF=8, FD=10 cm in the stated correspondence. Decide whether the figures are similar and state the length scale from ABC to DEF.
Construct your own complete figures, directed routes and corner correspondence. Enter every requested quantity, substitution and reason; missing outlines, dimensions and scale factors remain your own work.
Given panel 1 — drawing unit: cm
Blue: original supplied geometry. Amber: my actual submitted work. Each frame uses its stated drawing unit; compare mixed units by conversion, not by apparent screen size. Unlabelled sides and angles stay for calculation.
Both drawing axes have equal scale in cm. Whole-view turns rotate all geometry together; free points retain actual coordinates without snapping.
Triangle ABC has sides AB=3, BC=4, CA=5 cm. Triangle DEF has DE=6, EF=8, FD=10 cm in the stated correspondence. Decide whether the figures are similar and state the length scale from ABC to DEF.
Only literal givens are initially labelled. Every requested scalar, complete missing outline, chosen correspondence and explained reference remains learner work.
Compare other given figures in their stated units
Frames with the same unit and dimension share a drawing scale. Different units use separate labelled metric frames; convert physical lengths rather than comparing apparent screen sizes.
Given panel 2 — drawing unit: cm
Blue: original supplied geometry. Amber: my actual submitted work. Each frame uses its stated drawing unit; compare mixed units by conversion, not by apparent screen size. Unlabelled sides and angles stay for calculation.
Both drawing axes have equal scale in cm. Whole-view turns rotate all geometry together; free points retain actual coordinates without snapping.
Triangle ABC has sides AB=3, BC=4, CA=5 cm. Triangle DEF has DE=6, EF=8, FD=10 cm in the stated correspondence. Decide whether the figures are similar and state the length scale from ABC to DEF.
Only literal givens are initially labelled. Every requested scalar, complete missing outline, chosen correspondence and explained reference remains learner work.
Mark or construct the actual complete polygon in Given panel 1
No geometry is recorded for this action.
Enter your own count and names in boundary, route or anchor order. A cuboid needs all eight actual corners: base A–B–C–D and corresponding E–F–G–H, with all faces closing. Generic figure and solid drawings are coordinate witnesses, not extra hypotheses for the paper question.
This draft is incomplete: Enter your own integer count between 3 and 12. All entered raw text and points remain visible; no valid replacement figure is invented.
My quantities, equations and reasons
Enter exact supported fractions or decimals. A field asking for a coefficient of π takes the rational coefficient alone. Keep two decimal places where money is requested. Every response or unit edit closes previous feedback and references.
Required for the whole attempt. Give an exact supported scalar.
Required for the whole attempt.
Required for the whole attempt.
Written substitutions and reasons require human review; they are not automatically proved or marked.
Required for the whole attempt.
Written substitutions and reasons require human review; they are not automatically proved or marked.
This tool compares fixed mathematics with actual coordinate constructions. Written reasoning needs human review; it awards no marks, mastery, physical-instrument competence, specialist signoff or formal assessment approval. Mixed physical units require conversion; projected 3D angles and lengths are schematic.
Start with a correspondence
Similar figures have the same shape, although their sizes or positions may differ. First decide which vertex of one figure matches each vertex of the other. Write the pairs in a consistent order and use that order to match the sides between them. A side that happens to lie at the bottom of one picture need not match the bottom side of the other picture.
For each matched pair of sides, compare target length divided by original length, using the same length units. A single positive number must work for every corresponding side. Corresponding angles must also have the same magnitudes; for general polygons, matching side ratios alone is insufficient. Mark the vertex correspondence and write the direction of your scale comparison beside the figures; this makes an accidental change of direction much easier to spot.
Separate shape, size and orientation
A turn or a reflection changes where a figure appears, but it does not change its side lengths or angle magnitudes. Two similar figures may therefore be drawn with different orientations. The length ratio is still positive, including when one figure looks like a mirror image of the other. A signed enlargement factor or a centre of enlargement is a different transformation question; neither is needed to compare corresponding lengths here.
Congruent figures have the same shape and the same size. Their corresponding-length ratio is one. Similar figures with a different positive length ratio are not congruent. When you meet a concave outline, include its indentation edges and reflex angles in the correspondence. The bounding rectangle alone does not describe the whole shape, and replacing a reflex angle with its smaller exterior gap changes the angle being compared.
Check enough information
One matching length ratio is useful evidence, but it does not establish every other ratio for an arbitrary pair of figures. Nor do equal angles alone establish similarity for arbitrary polygons: rectangles can have different proportions while keeping all four right angles. Look for a stated similarity condition, a sufficient triangle condition, or the complete corresponding data needed for the particular figures.
For triangles, three corresponding proportional side lengths determine the same triangle shape; a carefully justified matching-angle argument can also establish similarity. State the actual condition you used rather than treating a roughly similar drawing as proof. Use a labelled sketch to organise the facts, and keep a numerical ratio check separate from any written justification that still needs human review.
Sources for this lesson
Pause and explain
A larger similar polygon is shown as a mirror image of a smaller one. What must still be true?
Worked example
Triangle ABC has sides AB=3, BC=4, CA=5 cm. Triangle DEF has DE=6, EF=8, FD=10 cm in the stated correspondence. Decide whether the figures are similar and state the length scale from ABC to DEF.
Show the worked solution
- Write the stated correspondence A↔D, B↔E and C↔F. This matches AB with DE, BC with EF and CA with FD; page position does not decide the pairs.
- Use target divided by original for all three sides: DE/AB=6/3=2, EF/BC=8/4=2 and FD/CA=10/5=2.
- Both sets form nondegenerate triangles. Their three corresponding side lengths have the same ratio, establishing the indicated triangle similarity and equal corresponding angles.
- The length factor from ABC to DEF is 2. It changes the size, so the figures are similar but are not congruent.
Answer The triangles are similar in the stated correspondence. The length factor from ABC to DEF is 2; corresponding angles remain equal.
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.
One right triangle has ordered side lengths 5, 12, 13 cm. A second triangle has corresponding lengths 10, 24, 26 cm and is shown in a rotated/reflected orientation. State the length scale from the first to the second and explain what happens to corresponding angles.
Give me a hint
Pair the sides in the supplied order before comparing every ratio.
Compare my reasoning
- Keep the sides paired in the supplied order, even though the second figure is drawn in a rotated/reflected orientation.
- Compare all three target-to-original ratios: 10/5=2, 24/12=2 and 26/13=2.
- The common side factor establishes the same right-triangle shape; corresponding angle magnitudes are unchanged.
- The second figure has twice the corresponding lengths. Rotation or reflection explains its orientation, not the change of size.
The first-to-second length scale is 2. Corresponding angles are unchanged; the different orientation does not alter that ratio.
Look for these in your work
- Check every stated corresponding side ratio.
- Explain why a different orientation leaves angle magnitudes unchanged.
- State the directed positive length factor.
One rectangle measures 2 by 3 cm; another measures 4 by 7 cm. All their angles are right angles. Are they similar? Show the decisive ratio check.
Give me a hint
Compare both rectangle dimension ratios; equal angles do not determine the side proportions.
Compare my reasoning
- Pair the shorter rectangle dimensions 2 cm and 4 cm, and the longer dimensions 3 cm and 7 cm.
- The target-to-original ratios are 4/2=2 and 7/3. These are not equal.
- Both rectangles have four right angles, but equal angles alone do not fix their side proportions.
- Turning either rectangle does not create a common corresponding side factor. The decisive length-ratio condition fails.
The rectangles are not similar: the corresponding ratios are 2 and 7/3, which differ despite the four right angles.
Look for these in your work
- Compare both corresponding rectangle ratios.
- Explain why the equal-angle information is insufficient.
- Give a justified similarity decision.
An L-shaped polygon is transformed by a uniform length factor 3/2 and then rotated. Explain what remains equal and how every corresponding length changes. Decide whether it is congruent to its original.
Give me a hint
Keep the indentation edges and the reflex corner in the correspondence.
Compare my reasoning
- Sketch a nondegenerate L outline and mark its corresponding corners, including the corner at the indentation.
- Every corresponding edge length is multiplied by 3/2, including the indentation edges. A following rotation does not change those new lengths.
- Corresponding angle magnitudes remain equal. In particular, the original reflex angle corresponds to a reflex angle of the same magnitude rather than to its smaller gap.
- The positive length factor 3/2 is not 1, so at least one nonzero side length changes and the new figure is not congruent to the original.
All corresponding angles, including reflex angles, remain equal and every corresponding length becomes 3/2 of its original. The figures are similar but not congruent.
Look for these in your work
- Include every boundary edge and its matching edge.
- Preserve corresponding reflex as well as ordinary angle magnitudes.
- Distinguish similar size changes from congruence.
Common mistakes
- Matching sides by page position instead of by their endpoint correspondence.
- Treating a reflected figure as having a negative corresponding-length ratio.
- Using equal angles alone to claim similarity for arbitrary quadrilaterals.
What can you explain now?
Can four right angles alone establish that two rectangles are similar? Explain.
Compare with the explanation
No. Their corresponding dimension ratios must also agree.
Right angles fix the corners but not the proportion between the two side lengths. Compare both corresponding length ratios, allowing a consistent change of orientation.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How would you write a vertex correspondence before checking similar figures, and which evidence would be insufficient?
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.