Describe rotation centres and signed angles
Recover a rotation centre and an object-to-image signed turn from named corresponding vertices, while recognising equivalent full-turn descriptions and identity ambiguity.
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
- Read labelled coordinates and join corresponding vertices.
- Use perpendicular bisectors and equal-distance reasoning.
- Interpret clockwise, anticlockwise and signed angles.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What information gives a complete ordinary rotation description?
What you will learn
- Use more than one corresponding vertex to determine and check a rotation centre.
- State an angle with its direction and the correct object-to-image order.
- Recognise angles differing by a whole turn and explain why an identity image does not recover a unique centre.
Polygon ABC has A(1,1), B(4,1), C(2,3). The corresponding image is A′(-1,1), B′(-1,4), C′(-3,2). The named image vertices correspond in the displayed order. Identify the transformation and give a complete object-to-image description; explain any equivalent angle descriptions.
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.
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.
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.
Begin with the named correspondence
A rotation keeps one centre fixed and turns every object point through the same angle about that centre. Match A with A′, B with B′ and the remaining labels before measuring a turn. Joining unrelated vertices can suggest a plausible movement that fails the stated correspondence.
For a moved pair P,P′, the centre is equally far from both points, so it lies on the perpendicular bisector of PP′. Use another moved pair to locate a common centre, then test all the remaining pairs. A vertex that stays fixed may be the centre; its zero joining segment supplies no perpendicular-bisector direction.
Read a signed turn from the centre
Draw rays from the recovered centre to one object vertex and its corresponding image vertex. Measure from the object ray towards the image ray, rather than in the reverse order. On the ordinary coordinate plane, anticlockwise angles are positive and clockwise angles are negative.
An angle needs its centre to give a complete rotation description. A quarter turn around one point usually gives a different image from the same turn around another. After describing the first pair, check that every other centre-to-vertex ray turns by the same signed angle and keeps its original radius.
Separate equivalent angles from missing information
Adding or subtracting a whole 360° turn leaves the final image unchanged. For example, a positive 40° turn and a negative 320° turn describe the same final rotation about one centre. A half turn may be stated as either +180° or −180°; both reach the opposite ray.
If every named point returns to itself, the identity can be described by a zero turn or a whole number of full turns about any centre. The image alone cannot recover a unique centre in that case. Keep this genuine ambiguity separate from an ordinary nonidentity rotation, whose complete labelled data can determine its centre.
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
Which description is equivalent to a +70° rotation about C?
Worked example
Polygon ABC has A(1,1), B(4,1), C(2,3). The corresponding image is A′(-1,1), B′(-1,4), C′(-3,2). The named image vertices correspond in the displayed order. Identify the transformation and give a complete object-to-image description; explain any equivalent angle descriptions.
Show the worked solution
- Use moved pairs AA′ and BB′. Their perpendicular bisectors meet at O=(0,0); check every remaining pair has equal distance from O. A fixed vertex supplies no nonzero joining-segment direction.
- From O, turn the ray to the object vertex towards its named image. The common signed angle is 90°. The same centre and turn map all the supplied labelled points, giving the complete description: rotation about (0,0) through 90° (anticlockwise for the stated sign).
- An equivalent angle is -270° about the same centre. Angles differing by 360° give the same final image; the inverse is rotation about (0,0) through -90° (clockwise for the stated sign). This nonidentity correspondence determines the stated pivot rather than an arbitrary identity centre.
Answer Rotation about (0,0) through 90° (anticlockwise for the stated sign); equivalent signed angle -270° about the same centre.
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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). The corresponding image is A′(3,1), B′(3,-2), C′(5,-3), D′(6,0). The named image vertices correspond in the displayed order. Identify the transformation and give a complete object-to-image description; explain any equivalent angle descriptions.
Give me a hint
Use moved pairs for a common pivot, then read the object-to-image signed turn.
Compare my reasoning
- Use moved pairs AA′ and BB′. Their perpendicular bisectors meet at O=(2,-1); check every remaining pair has equal distance from O. A fixed vertex supplies no nonzero joining-segment direction.
- From O, turn the ray to the object vertex towards its named image. The common signed angle is -90°. The same centre and turn map all the supplied labelled points, giving the complete description: rotation about (2,-1) through -90° (clockwise for the stated sign).
- An equivalent angle is 270° about the same centre. Angles differing by 360° give the same final image; the inverse is rotation about (2,-1) through 90° (anticlockwise for the stated sign). This nonidentity correspondence determines the stated pivot rather than an arbitrary identity centre.
Rotation about (2,-1) through -90° (clockwise for the stated sign); equivalent signed angle 270° about the same centre.
Look for these in your work
- Use more than one corresponding vertex to determine and check a rotation centre.
- State an angle with its direction and the correct object-to-image order.
- Recognise angles differing by a whole turn and explain why an identity image does not recover a unique centre.
Polygon ABCDEF has A(0,0), B(4,0), C(4,1), D(1,1), E(1,3), F(0,3). The corresponding image is A′(-2,4), B′(-6,4), C′(-6,3), D′(-3,3), E′(-3,1), F′(-2,1). The named image vertices correspond in the displayed order. Identify the transformation and give a complete object-to-image description; explain any equivalent angle descriptions.
Give me a hint
Use moved pairs for a common pivot, then read the object-to-image signed turn.
Compare my reasoning
- Use moved pairs AA′ and BB′. Their perpendicular bisectors meet at O=(-1,2); check every remaining pair has equal distance from O. A fixed vertex supplies no nonzero joining-segment direction.
- From O, turn the ray to the object vertex towards its named image. The common signed angle is 180°. The same centre and turn map all the supplied labelled points, giving the complete description: rotation about (-1,2) through 180° (anticlockwise for the stated sign).
- An equivalent angle is -180° about the same centre. Angles differing by 360° give the same final image; the inverse is rotation about (-1,2) through -180° (clockwise for the stated sign). This nonidentity correspondence determines the stated pivot rather than an arbitrary identity centre.
Rotation about (-1,2) through 180° (anticlockwise for the stated sign); equivalent signed angle -180° about the same centre.
Look for these in your work
- Use more than one corresponding vertex to determine and check a rotation centre.
- State an angle with its direction and the correct object-to-image order.
- Recognise angles differing by a whole turn and explain why an identity image does not recover a unique centre.
Polygon ABCD has A(-2,1), B(1,1), C(2,4), D(-1,3). The corresponding image is A′(1,4), B′(1,1), C′(4,0), D′(3,3). The named image vertices correspond in the displayed order. Identify the transformation and give a complete object-to-image description; explain any equivalent angle descriptions.
Give me a hint
Use moved pairs for a common pivot, then read the object-to-image signed turn.
Compare my reasoning
- Use moved pairs AA′ and CC′. Their perpendicular bisectors meet at O=(1,1); check every remaining pair has equal distance from O. A fixed vertex supplies no nonzero joining-segment direction.
- From O, turn the ray to the object vertex towards its named image. The common signed angle is 270°. The same centre and turn map all the supplied labelled points, giving the complete description: rotation about (1,1) through 270° (anticlockwise for the stated sign).
- An equivalent angle is -90° about the same centre. Angles differing by 360° give the same final image; the inverse is rotation about (1,1) through -270° (clockwise for the stated sign). This nonidentity correspondence determines the stated pivot rather than an arbitrary identity centre.
Rotation about (1,1) through 270° (anticlockwise for the stated sign); equivalent signed angle -90° about the same centre.
Look for these in your work
- Use more than one corresponding vertex to determine and check a rotation centre.
- State an angle with its direction and the correct object-to-image order.
- Recognise angles differing by a whole turn and explain why an identity image does not recover a unique centre.
Common mistakes
- Measuring image-to-object instead of object-to-image.
- Trying to bisect a zero segment from a fixed vertex.
- Rejecting an equivalent angle or inventing a unique centre for an identity.
What can you explain now?
Can one fixed vertex on an unchanged image determine the centre of a full-turn rotation?
Compare with the explanation
No. A full turn about any centre gives the same identity image.
A fixed point supplies no nonzero joining segment, and the identity does not single out a rotation centre.
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Make it stick
Explain how moved corresponding pairs determine a centre, then give an equivalent signed angle and describe the identity 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.