Verify congruence under rigid transformations
Verify corresponding lengths and interior angles under rotations, reflections and translations, and distinguish a genuine rigid image from an enlargement that merely keeps angles.
Exam scope and assessment notes
These twelve shared lessons apply to Foundation and inherited Higher for F5.2A–M; no extra Higher row is added. Keep named corners, signed turns, equivalent mirror equations, exact fractions, positive factors and the freely chosen-centre and identity exceptions. General-angle coordinates are final3dp approximations of unrounded geometry. Written explanations need human review; coordinate agreement does not certify proof or physical construction. Linear November2026/June2027 entry and regional-centre rules remain separate from modular awards. Book2 Shape and Space7 tags and Hubs66–71 are navigation only: map223–248 differs from proof222/summary247/next248, with chapter/Hub bodies unavailable.
Before you begin
- Match corresponding polygon sides and interior angles.
- Use exact distances or squared distances between coordinates.
- Distinguish rotations, reflections, translations and enlargements.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which statement is sufficient for congruence in these transformations?
What you will learn
- Compare every corresponding side and interior angle.
- Distinguish orientation reversal from a change of size or angle.
- Explain congruence for rigid maps and contrast a nonidentity enlargement.
Polygon ABC has A(1,1), B(4,1), C(2,3). In the given order, rotate about (2,-1) by 90 degrees, using positive for anticlockwise. Construct the named image and compare every corresponding side length and angle. Decide whether the figures remain congruent and explain the identity-factor exception where relevant.
Describe your operation, construct every named image point, and record your actual directed traces. Only the question supplies the givens. The board does not transform a shape or recover a missing parameter for you.
Both Cartesian directions have equal drawing scale. Blue is supplied geometry, amber is my actual work, purple is dirty retained work, and green is a separately gated reference. Screen size is not physical instrument calibration.
Actual boundaries retain their entered order, closure, concavity and wrong or incomplete points. No reflected corner, rotation endpoint, mirror foot or target polygon is filled automatically. Construction rays continue beyond their anchors; mirror lines continue both ways; joining segments keep their original endpoints.
Some actual points are outside this viewport and remain unchanged. The frame clips the drawing, not its mathematical domain. Pan or zoom to inspect them; no artificial arrowhead is placed at a crop intersection.
Supplied rotation 90° about (2, -1). This description does not generate a target boundary.
This finite fixed-case comparison is not proof marking, marks, mastery, physical construction calibration or specialist/formal assessment approval. General-angle points stay unrounded; a separately requested three-decimal written table is an approximation.
Actual operation 1
No actual construction is recorded for this action.
Choose a point slot before adding a keyboard coordinate or picking on the board. Named picks retain exact given or current clean recorded coordinates. Free picks retain their unrounded actual planar coordinates; they do not snap to an expected result.
My written coordinates, quantities and reasoning
Write both coordinates as x,y; exact fractions and finite decimals are supported.
Write both coordinates as x,y; exact fractions and finite decimals are supported.
Write both coordinates as x,y; exact fractions and finite decimals are supported.
Use a nonnegative exact fraction, decimal or supported single-radical length. Select the stated drawing unit.
Use a nonnegative exact fraction, decimal or supported single-radical length. Select the stated drawing unit.
Choose Yes or No after checking your current construction.
Use a nonnegative exact fraction, decimal or supported single-radical length. Select the stated drawing unit.
Use a nonnegative exact fraction, decimal or supported single-radical length. Select the stated drawing unit.
Choose Yes or No after checking your current construction.
Use a nonnegative exact fraction, decimal or supported single-radical length. Select the stated drawing unit.
Use a nonnegative exact fraction, decimal or supported single-radical length. Select the stated drawing unit.
Choose Yes or No after checking your current construction.
Enter your exact fraction or finite decimal, using the stated conditions.
Choose Yes or No after checking your current construction.
Explain your actual correspondence, construction and conditions. A written attempt is not an automatically verified proof.
Explain your actual correspondence, construction and conditions. A written attempt is not an automatically verified proof.
Explain your actual correspondence, construction and conditions. A written attempt is not an automatically verified proof.
This sequence is optional. If you add operations, complete every chosen parameter or remove the sequence. The tool generates no inverse for you.
These original cases support mathematical self-checking. They do not award marks, mastery, proof approval, physical construction competence, specialist signoff or formal assessment readiness. Positive scale factors only; unchanged images do not uniquely recover a centre. Keep general-angle construction coordinates unrounded and separately requested written coordinates at three decimal places.
Compare corresponding features, not a silhouette
Rotations, reflections and translations preserve distances between every pair of points. Consequently, each image side has the length of its named corresponding object side. Match AB with A′B′, BC with B′C′ and the remaining sides, instead of comparing whichever sides look closest in the drawing.
These transformations also preserve interior angles, so the image is congruent to the object. A numerical check of all side pairs and angle correspondences can support the calculation in a particular example, while the explanation should identify the distance-preserving transformation that gives the general reason.
A mirror can reverse orientation while preserving angles
Reflection changes the clockwise or anticlockwise sense of a boundary traversal. That reversal does not replace an interior angle by its supplement or change a reflex corner into an ordinary one. Measure the angle inside the actual polygon boundary, including its concave notch, and compare it with the corresponding interior angle.
Rotations and translations preserve the traversal sense, while all three rigid transformations keep the same shape and size. A figure may overlap the object or share a fixed vertex without losing congruence. The named correspondence, not the amount of separation on the page, determines which features are compared.
Use enlargement as a contrast
A positive enlargement preserves corresponding angles but multiplies all corresponding lengths by its factor k. For a nondegenerate polygon, k≠1 therefore changes size and prevents congruence even though the angles agree. Equal angles alone do not establish equal size.
At k=1 the enlargement is the identity and lengths are unchanged, so the figures are congruent. Keep that exception explicit rather than claiming that every enlargement changes every length. Explain the particular factor or rigid map used; agreement in a fixed example does not certify a general written proof or a practical construction.
Sources for this lesson
- Pearson Mathematics A linear specification · Issue2 · Transformation geometry Foundation printed23/physical29 and Higher inheritance printed38/physical44
- Pearson 2026/2027 International GCSE entry manual · centre regions physical3 and Mathematics A physical25
- Pearson literal second-edition book/Hub map · Book2 Shape and Space7 physical12 · navigation only
Pause and explain
A nondegenerate figure is enlarged by positive factor 3/4. What follows?
Worked example
Polygon ABC has A(1,1), B(4,1), C(2,3). In the given order, rotate about (2,-1) by 90 degrees, using positive for anticlockwise. Construct the named image and compare every corresponding side length and angle. Decide whether the figures remain congruent and explain the identity-factor exception where relevant.
Show the worked solution
- Construct the full image using rotation about (2,-1) through 90° (anticlockwise for the stated sign): A′=(0,-2), B′=(0,1), C′=(-2,-1). Preserve the given names and boundary order rather than comparing unrelated sides.
- Compare all corresponding side lengths exactly: AB: 3 → 3; BC: 2√2 → 2√2; CA: √5 → √5, in coordinate units. The common length factor is 1; squared distances therefore have factor 1.
- All corresponding interior angles agree: ∠A′=∠A, ∠B′=∠B, ∠C′=∠C. The same rule covers reflex interior angles whenever a boundary is concave; the boundary keeps its corresponding interior sides. Because k=1, size is unchanged and the image is congruent to the object.
- The inverse is rotation about (2,-1) through -90° (clockwise for the stated sign). The identity factor 1 is a valid positive exception to length change, but does not make a nonunit enlargement congruent. Numerical agreement here does not approve a general proof or practical construction.
Answer A′=(0,-2), B′=(0,1), C′=(-2,-1). AB: 3 → 3; BC: 2√2 → 2√2; CA: √5 → √5. ∠A′=∠A, ∠B′=∠B, ∠C′=∠C. Congruent: all corresponding lengths and angles are unchanged.
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.
Polygon ABCD has A(0,0), B(3,0), C(4,2), D(1,3). In the given order, reflect in the line 1x+(1)y=3. Construct the named image and compare every corresponding side length and angle. Decide whether the figures remain congruent and explain the identity-factor exception where relevant.
Give me a hint
Compare all corresponding side lengths and interior angles; orientation reversal is separate from congruence.
Compare my reasoning
- Construct the full image using reflection in x+y=3: A′=(3,3), B′=(3,0), C′=(1,-1), D′=(0,2). Preserve the given names and boundary order rather than comparing unrelated sides.
- Compare all corresponding side lengths exactly: AB: 3 → 3; BC: √5 → √5; CD: √10 → √10; DA: √10 → √10, in coordinate units. The common length factor is 1; squared distances therefore have factor 1.
- All corresponding interior angles agree: ∠A′=∠A, ∠B′=∠B, ∠C′=∠C, ∠D′=∠D. The same rule covers reflex interior angles whenever a boundary is concave; reflection reverses orientation without changing those angle measures. Because k=1, size is unchanged and the image is congruent to the object.
- The inverse is reflection in x+y=3. The identity factor 1 is a valid positive exception to length change, but does not make a nonunit enlargement congruent. Numerical agreement here does not approve a general proof or practical construction.
A′=(3,3), B′=(3,0), C′=(1,-1), D′=(0,2). AB: 3 → 3; BC: √5 → √5; CD: √10 → √10; DA: √10 → √10. ∠A′=∠A, ∠B′=∠B, ∠C′=∠C, ∠D′=∠D. Congruent: all corresponding lengths and angles are unchanged.
Look for these in your work
- Compare every corresponding side and interior angle.
- Distinguish orientation reversal from a change of size or angle.
- Explain congruence for rigid maps and contrast a nonidentity enlargement.
Polygon ABCDEF has A(0,0), B(4,0), C(4,1), D(1,1), E(1,3), F(0,3). In the given order, translate by the column vector with top entry -3 and bottom entry 4. Construct the named image and compare every corresponding side length and angle. Decide whether the figures remain congruent and explain the identity-factor exception where relevant.
Give me a hint
Compare all corresponding side lengths and interior angles; orientation reversal is separate from congruence.
Compare my reasoning
- Construct the full image using translation by column[-3;4]: A′=(-3,4), B′=(1,4), C′=(1,5), D′=(-2,5), E′=(-2,7), F′=(-3,7). Preserve the given names and boundary order rather than comparing unrelated sides.
- Compare all corresponding side lengths exactly: AB: 4 → 4; BC: 1 → 1; CD: 3 → 3; DE: 2 → 2; EF: 1 → 1; FA: 3 → 3, in coordinate units. The common length factor is 1; squared distances therefore have factor 1.
- All corresponding interior angles agree: ∠A′=∠A, ∠B′=∠B, ∠C′=∠C, ∠D′=∠D, ∠E′=∠E, ∠F′=∠F. The same rule covers reflex interior angles whenever a boundary is concave; the boundary keeps its corresponding interior sides. Because k=1, size is unchanged and the image is congruent to the object.
- The inverse is translation by column[3;-4]. The identity factor 1 is a valid positive exception to length change, but does not make a nonunit enlargement congruent. Numerical agreement here does not approve a general proof or practical construction.
A′=(-3,4), B′=(1,4), C′=(1,5), D′=(-2,5), E′=(-2,7), F′=(-3,7). AB: 4 → 4; BC: 1 → 1; CD: 3 → 3; DE: 2 → 2; EF: 1 → 1; FA: 3 → 3. ∠A′=∠A, ∠B′=∠B, ∠C′=∠C, ∠D′=∠D, ∠E′=∠E, ∠F′=∠F. Congruent: all corresponding lengths and angles are unchanged.
Look for these in your work
- Compare every corresponding side and interior angle.
- Distinguish orientation reversal from a change of size or angle.
- Explain congruence for rigid maps and contrast a nonidentity enlargement.
Polygon ABCD has A(-2,1), B(1,1), C(2,4), D(-1,3). In the given order, enlarge about (1,0) with positive factor 2. Construct the named image and compare every corresponding side length and angle. Decide whether the figures remain congruent and explain the identity-factor exception where relevant.
Give me a hint
Compare all corresponding side lengths and interior angles; orientation reversal is separate from congruence.
Compare my reasoning
- Construct the full image using enlargement about (1,0) with positive factor 2: A′=(-5,2), B′=(1,2), C′=(3,8), D′=(-3,6). Preserve the given names and boundary order rather than comparing unrelated sides.
- Compare all corresponding side lengths exactly: AB: 3 → 6; BC: √10 → 2√10; CD: √10 → 2√10; DA: √5 → 2√5, in coordinate units. The common length factor is 2; squared distances therefore have factor 4.
- All corresponding interior angles agree: ∠A′=∠A, ∠B′=∠B, ∠C′=∠C, ∠D′=∠D. The same rule covers reflex interior angles whenever a boundary is concave; the boundary keeps its corresponding interior sides. Because the nonunit factor 2 changes nonzero side lengths, equal angles alone do not make this image congruent.
- The inverse is enlargement about (1,0) with positive factor 1/2. The identity factor 1 is a valid positive exception to length change, but does not make a nonunit enlargement congruent. Numerical agreement here does not approve a general proof or practical construction.
A′=(-5,2), B′=(1,2), C′=(3,8), D′=(-3,6). AB: 3 → 6; BC: √10 → 2√10; CD: √10 → 2√10; DA: √5 → 2√5. ∠A′=∠A, ∠B′=∠B, ∠C′=∠C, ∠D′=∠D. Not congruent: angles are unchanged but corresponding lengths scale by 2.
Look for these in your work
- Compare every corresponding side and interior angle.
- Distinguish orientation reversal from a change of size or angle.
- Explain congruence for rigid maps and contrast a nonidentity enlargement.
Common mistakes
- Using equal angles alone to claim congruence.
- Treating reflected reflex angles as their ordinary complements.
- Claiming every positive enlargement changes size, including factor 1.
What can you explain now?
A reflected concave polygon has a 240° interior corner. Is its image corner 120°?
Compare with the explanation
No. The corresponding interior angle is still 240°.
The orientation reverses, but the polygon’s interior and concave notch are reflected together; the smaller 120° sweep is not that interior angle.
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Make it stick
Explain why the three rigid maps preserve congruence, how reflection changes orientation and why a factor 1 enlargement is an exception.
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.