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Combinations of transformations and invariance worksheet — GCSE Higher
Fifteen questions on "combinations of transformations and invariance" — DfE statement G8. Print it, or print three versions so neighbours cannot copy by letter; the key gives the letter for each version.
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Combinations of transformations and invariance worksheet — GCSE Higher
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- 1.A game designer places a coin at (5, 3) on a grid. The game rotates the coin 90° clockwise about the point (2, 3) each time the player presses a button, and this is applied twice in a row. Work out the coordinates of the coin after the button is pressed twice.
- 2.Point P has coordinates (6, 4). P is rotated 90° clockwise about the point (1, 2), and the image is then translated by the vector (−3, 5). Work out the coordinates of the final image of P.
- 3.A line passes through the points (1, 2) and (3, 6). The line is reflected in the x-axis, and the image is then translated by the vector (2, 0). Work out the gradient of the image line after both transformations.
- 4.Point P has coordinates (4, 2). P is rotated 90° clockwise about the point (1, 1), and the image is then reflected in the line x = 1. Work out the coordinates of the final image of P.
- 5.Point P has coordinates (2, 1). Transformation A reflects a point in the x-axis. Transformation B translates a point by the vector (0, 4). Work out the coordinates of the image of P when A is applied first, followed by B.
- 6.Triangle T has vertices (1, 1), (3, 1) and (1, 4). It is mapped onto triangle T′ with vertices (5, −1), (3, −1) and (5, −4). Which single composition of two transformations maps T onto T′?
- 7.Square B has vertices (1, 1), (4, 1), (4, 4) and (1, 4). It is rotated 90° clockwise about the origin, and the image is then translated by the vector (2, −1). Work out the coordinates of the image of (4, 1).
- 8.Triangle D has vertices (3, 1), (3, 5) and (6, 1). It is reflected in the line x = 3. Write down the number of vertices of the triangle that are invariant under this reflection.
- 9.Triangle T has an area of 6 cm². It is enlarged by scale factor −2 about the origin, and the image is then rotated 90° about the origin. Work out the area of the final image, and state whether it is the same way round as T or a mirror image of it.
- 10.A shape is translated by the vector (5, −3), and the image is then translated by the vector (−5, 3). Which single transformation has the same overall effect as this combination, for every point?
- 11.Line l1 has equation x = 2, and line l2 has equation y = x − 1. They intersect at a single point. A shape is reflected in l1, and the image is then reflected in l2. Which point is invariant under this combined transformation?y = x − 1
- 12.A trapezium has vertices (0, 2), (4, 2), (3, 5) and (1, 5). It is rotated 180° about the point (2, 2), and the image is then translated by the vector (1, −5). Work out the coordinates of the image of (0, 2).
- 13.Triangle A has vertices (2, 1), (5, 1) and (2, 4). It is translated by the vector (−3, 2) and the image is then reflected in the x-axis. Work out the coordinates of the image of (5, 1).
- 14.A trapezium has vertices (2, 1), (6, 1), (5, 4) and (3, 4). It is rotated 180° about the vertex (2, 1). Work out the number of points on the trapezium — including its vertices, edges and interior — that are invariant under this rotation.
- 15.Point Q has coordinates (3, 5). It is translated by the vector (1, −2), and the image is then enlarged by scale factor 3, centre the origin. Work out the coordinates of the final image of Q.
Answer key
- (d) (−1, 3) — Method: two 90° rotations about the SAME centre, applied one after another, combine into a single 180° rotation about that same centre: use the shortcut (x, y) → (2a − x, 2b − y) for a half-turn about (a, b). Working: with centre (2, 3), doubling each coordinate gives 2 × 2 = 4 and 2 × 3 = 6, so the rule is (x, y) → (4 − x, 6 − y). Applying it to (5, 3) gives 4 − 5 = −1 and 6 − 3 = 3, so the coin ends at (−1, 3). Answer: (−1, 3). Rotate about the centre (2, 3) stated in the game, not about the origin, and remember the button is pressed TWICE: stopping after one press, or rotating about the wrong centre, both leave the coin somewhere else.
- (b) (0, 2) — Method: to rotate about a point that is not the origin, first subtract the centre's coordinates, apply the rotation rule to the shifted point, then add the centre's coordinates back on; only after that do you apply the translation, in the order the question states them. Working: shifting P relative to the centre gives (6 − 1, 4 − 2) = (5, 2); rotating 90° clockwise sends (x, y) to (y, −x), giving (2, −5); adding the centre back on gives (2 + 1, −5 + 2) = (3, −3); applying the translation (−3, 5) gives (3 − 3, −3 + 5) = (0, 2). Answer: (0, 2). Applying the translation BEFORE the rotation, reversing the order the question gives them in, gives (8, 0); stopping after the rotation and forgetting the translation altogether gives (3, −3); and rotating anticlockwise instead of clockwise, using (x, y) → (−y, x), gives (−4, 12). Always carry out the two transformations in the order stated — rotate about the given centre first, then translate — and check each step before moving to the next.
- (b) −2 — The gradient of the original line is (6 − 2) ÷ (3 − 1) = 4 ÷ 2 = 2. Reflecting in the x-axis sends every y-coordinate to its negative, which flips the sign of the gradient: the image line has gradient −2. Translating by (2, 0) is a horizontal shift, which does not change the line's steepness or direction at all, so the gradient stays at −2. Assuming the gradient is unaffected by the reflection gives 2, the original gradient carried straight through. Thinking a reflection in the x-axis turns a gradient into its positive reciprocal gives 1/2. Combining that same wrong idea with the sign flip from the reflection gives −1/2. Only the sign flips, from the reflection, and translating never changes a gradient at all, so the answer is −2.
- (d) (0, −2) — To rotate (4, 2) by 90° clockwise about (1, 1), first find its position relative to the centre: (4 − 1, 2 − 1) = (3, 1). A 90° clockwise rotation sends (a, b) to (b, −a), so (3, 1) becomes (1, −3); adding the centre back gives (1 + 1, 1 − 3) = (2, −2). Reflecting (2, −2) in the line x = 1 gives (2 × 1 − 2, −2) = (0, −2). Doing the two transformations in the opposite order, reflecting first and then rotating, gives a different result, (2, 4), which shows the order matters. Stopping after the rotation and forgetting the reflection gives (2, −2). Stopping after only reflecting P in x = 1 and forgetting the rotation entirely gives (−2, 2). Rotate first, then reflect, in that order, and the final image is (0, −2).
- (b) (2, 3) — Applying A first: reflecting (2, 1) in the x-axis gives (2, −1). Applying B to that image: translating (2, −1) by (0, 4) gives (2, −1 + 4) = (2, 3). Applying the transformations in the opposite order — B first, then A — gives a different result: (2, 1) translates to (2, 5), which then reflects to (2, −5); this shows that the order genuinely matters here. Applying only A and stopping there, without the translation, gives (2, −1). Applying only B and stopping there, without the reflection, gives (2, 5). Do both transformations, in the order A then B, and the image of P is (2, 3).
- (b) Rotate 180° about the origin, then translate by (6, 0). — Rotating 180° about the origin sends (x, y) to (−x, −y); applied to T's vertices (1, 1), (3, 1) and (1, 4) this gives (−1, −1), (−3, −1) and (−1, −4). Translating this image by the vector (6, 0) adds 6 to every x-coordinate, giving (5, −1), (3, −1) and (5, −4), which matches T′ exactly. Reflecting in the x-axis first changes the sign of the y-coordinate only, and translating that image by (6, 0) gives (7, −1), (9, −1) and (7, −4) — the wrong triangle. Using the correct rotation but translating by (4, 0) instead of (6, 0) gives (3, −1), (1, −1) and (3, −4), shifted 2 units too far left. Reflecting in the y-axis first changes the sign of the x-coordinate only, so translating that image by (6, 0) leaves every y-coordinate positive, giving (5, 1), (3, 1) and (5, 4) — the correct x-coordinates but the wrong sign throughout on y.
- (b) (3, −5) — Method: apply the rotation to the point first, then translate the image, in the order the question gives them. Working: rotating (4, 1) by 90° clockwise about the origin sends (x, y) to (y, −x), so (4, 1) becomes (1, −4). Translating (1, −4) by the vector (2, −1) gives 1 + 2 = 3 and −4 − 1 = −5, so the final image is (3, −5). Answer: (3, −5). Use the CLOCKWISE rule, (x, y) → (y, −x), not the anticlockwise one, and apply the rotation before the translation, exactly as the question states them: reversing the order or the direction of turn both land on a different point.
- (d) 2 — Method: a point is invariant under a reflection exactly when it lies on the mirror line, so check each vertex's coordinate against the line's equation. Working: the line of reflection is x = 3, so a vertex is invariant only if its x-coordinate equals 3. (3, 1) has x = 3, so it is invariant. (3, 5) has x = 3, so it is also invariant. (6, 1) has x = 6, so it is not invariant, since 6 is not equal to 3. Two of the three vertices are invariant. Answer: 2. Check EVERY vertex against the mirror line's equation rather than assuming a shape either keeps all its vertices fixed or none of them: a vertex is invariant only when it sits exactly on the line, and here two do and one does not.
- (d) 24 cm²; same way round as T — An enlargement scales area by the square of the scale factor, whatever its sign: area factor = (−2)² = 4, so the image's area is 6 × 4 = 24 cm². A negative scale factor is equivalent to an enlargement by the positive scale factor together with a rotation of 180°, so it does not create a mirror image — it is still a direct transformation, and combining it with a further rotation cannot create one either, since rotations never change a shape's orientation. Using the scale factor itself rather than its square gives area 6 × 2 = 12 cm², which is too small. Believing that a negative scale factor reflects the shape gives 'a mirror image of T' alongside the correct area of 24 cm², and combining both mistakes gives 12 cm² together with 'a mirror image of T'.
- (d) No transformation — every point stays exactly where it was — The two vectors (5, −3) and (−5, 3) are opposites, so adding them gives (0, 0): every point ends up exactly where it started, and there is no transformation at all. Misreading the second vector's signs and effectively adding (5, −3) to itself instead of to its opposite gives a translation by the vector (10, −6). Assuming two translations must combine into a reflection gives a reflection in the x-axis — but a reflection reverses orientation, and translations never do. Assuming that two opposite vectors must mean a half turn gives a rotation of 180° about the origin — but a 180° rotation moves every point except its own centre, whereas this pair of translations leaves every single point exactly where it was. Two translations by opposite vectors always cancel exactly, leaving every point unmoved.
- (c) (2, 1) — A point that lies on both mirror lines is fixed by each reflection individually, and so is fixed by the combination of the two — it is the intersection point of l1 and l2 that is invariant. Substituting x = 2 into y = x − 1 gives y = 2 − 1 = 1, so the intersection point is (2, 1). Forgetting the '− 1' in l2's equation and using y = x instead gives (2, 2). Making a sign error and computing y = x − (−1) = x + 1 instead gives (2, 3). Solving for x from an assumed y = 0 instead of substituting the given x = 2 gives (1, 0). Substitute x = 2 into l2's equation correctly, and the invariant point is (2, 1).
- (b) (5, −3) — Method: rotate the point about the given centre first, then translate the image, in the stated order. Working: rotating (0, 2) by 180° about (2, 2) uses the rule (x, y) → (4 − x, 4 − y), since the centre doubles in each coordinate. This gives 4 − 0 = 4 and 4 − 2 = 2, so (0, 2) maps to (4, 2). Translating (4, 2) by the vector (1, −5) gives 4 + 1 = 5 and 2 − 5 = −3, so the final image is (5, −3). Answer: (5, −3). Rotate about the centre GIVEN in the question, (2, 2), not about the origin, and translate the rotated image afterwards, in that order: rotating about the wrong centre, swapping the order, or stopping after one step all give a different point.
- (c) (2, −3) — Method: apply the transformations in the order given, translation first, then reflection, to the point itself. Working: translating (5, 1) by the vector (−3, 2) moves the x-coordinate by −3 and the y-coordinate by 2, giving 5 − 3 = 2 and 1 + 2 = 3, so the image after the translation is (2, 3). Reflecting this point in the x-axis leaves the x-coordinate unchanged and reverses the sign of the y-coordinate, giving (2, −3). Answer: (2, −3). Do the two maps in the stated order and carry the point all the way through both of them: swapping the order, or stopping after the first map, lands you on a different point altogether.
- (b) 1 point — A rotation about a point P always leaves P itself unchanged, and — unless the shape has rotational symmetry about that exact point — no other point of the shape maps onto itself. The centre of rotation here is the vertex (2, 1), so exactly one point of the trapezium, that vertex, is invariant. Thinking that a rotation always has no invariant points ignores the centre of rotation itself, which is always fixed, and gives 0 points. Assuming every vertex of the shape is invariant confuses a rotation with the identity transformation and gives 4 points, the total number of vertices. Believing that the point diametrically opposite the centre is also fixed applies point symmetry of the whole coordinate grid rather than checking whether that point actually lies on this particular trapezium, and gives 2 points.
- (b) (12, 9) — Apply the transformations in the order given: first translate, then enlarge. Translating (3, 5) by the vector (1, −2) gives (3 + 1, 5 − 2) = (4, 3). Enlarging this by scale factor 3 about the origin multiplies both coordinates by 3: (4 × 3, 3 × 3) = (12, 9). Enlarging first and translating afterwards reverses the order and gives (3 × 3 + 1, 5 × 3 − 2) = (10, 13), a different point because the two transformations do not commute. Enlarging the original point by scale factor 3 while forgetting to translate it at all gives (3 × 3, 5 × 3) = (9, 15). Reversing the signs of the translation vector before applying it gives (3 − 1, 5 + 2) = (2, 7), which then enlarges to (2 × 3, 7 × 3) = (6, 21).
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