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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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Answer key: Combinations of transformations and invariance worksheet — GCSE Higher
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- (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.
- (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).
- (d) y = 2 — Two reflections in perpendicular lines that cross at a point combine to a 180° rotation about that point. The line x = 3 is vertical, so the second line must be horizontal, and it must pass through the centre of rotation (3, 2) — that line is y = 2. Taking the y-coordinate of the centre but writing it against the wrong letter gives y = 3. Assuming the second line must also be vertical, like the first one, and just swapping in the other coordinate gives x = 2. Reaching for the standard mirror line y = x without checking that it actually passes through (3, 2) gives y = x — it does not pass through that point at all. The line that is both perpendicular to x = 3 and through (3, 2) is y = 2.
- (d) A translation by the vector (8, 0) — Method: two reflections in PARALLEL lines combine into a single translation, perpendicular to the lines, of size twice the distance between them; two reflections in lines that CROSS combine into a rotation instead, never a translation. Working: the lines x = 2 and x = 6 are parallel, a distance of 6 − 2 = 4 apart. Doubling this distance gives 2 × 4 = 8, and the translation runs in the direction from the first line towards the second, so the vector is (8, 0). Answer: a translation by the vector (8, 0). Double the distance between the lines rather than using it directly, keep the direction running from the FIRST line reflected to the SECOND, and remember that two reflections in lines that never meet can only give a translation, never a rotation.
- (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).
- (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).
- (d) (−14, 5) — To undo a composition, reverse the order and invert each transformation: undo the rotation first, then undo the translation. The inverse of 'rotate 90° clockwise about the origin' is 'rotate 90° anticlockwise about the origin', which maps (x, y) to (−y, x); applied to (3, 9) this gives (−9, 3). Then undo the translation by subtracting the vector (5, −2), i.e. adding (−5, 2): (−9 − 5, 3 + 2) = (−14, 5). Undoing the two inverse steps in the same order as the original composition, rather than reversing it, gives (−11, −2). Rotating 90° clockwise again instead of inverting the rotation's direction gives (4, −1). Adding the translation vector again instead of subtracting it, after correctly inverting the rotation, gives (−4, 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.
- (d) (4, −2) — Method: apply the rotation to the point first, then reflect the rotated image, in the order stated. Working: rotating (4, 2) by 90° anticlockwise about the origin sends (x, y) to (−y, x), so (4, 2) becomes (−2, 4). Reflecting (−2, 4) in the line y = x swaps its coordinates, giving (4, −2). Answer: (4, −2). Rotate before you reflect, exactly as the question orders them: these two maps do not commute, so reflecting first, only rotating without swapping the coordinates afterwards, or forgetting to negate the coordinate when rotating anticlockwise all send you to a different point.
- (a) Translation by the vector (0, 6) — Method: reflecting twice in two parallel horizontal lines is always equivalent to a single translation, at right angles to the lines, of twice the distance between them. Working: the two lines are 4 − 1 = 3 units apart, so the translation is 2 × 3 = 6 units in the positive y-direction. Answer: translation by the vector (0, 6). Using just the gap itself, without doubling it, gives (0, 3); translating in the negative y-direction, from the second line back towards the first, gives (0, −6); and describing the combination as a single reflection in the line halfway between them, y = 2.5, confuses this combination with the effect of a single reflection — two reflections in parallel lines are always equivalent to a translation, never to another reflection. Always double the gap between the lines, and translate in the direction from the first line towards the second.
- (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) Translation by the vector (8, 0) — Method: reflecting twice in two parallel vertical lines is always equivalent to a single translation, at right angles to the lines, of twice the distance between them. Working: the two lines are 5 − 1 = 4 units apart, so the translation is 2 × 4 = 8 units in the positive x-direction. Answer: translation by the vector (8, 0). Using just the gap itself, without doubling it, gives (4, 0); translating in the negative x-direction, from the second line back towards the first, gives (−8, 0); and describing the combination as a single reflection in the line halfway between them, x = 3, confuses this combination with the effect of a single reflection — two reflections in parallel lines are always equivalent to a translation, never to another reflection. Always double the gap between the lines, and translate in the direction from the first line towards the second.
- (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.
- (a) A rotation of 180° about the origin — Method: composing two reflections in lines that cross is always a single rotation about the point where the lines meet, through twice the angle between them. Working: the x-axis and y-axis meet at the origin at an angle of 90°, so the combined transformation is a rotation about the origin through 2 × 90 = 180 degrees. Answer: a rotation of 180° about the origin. The rotation angle is TWICE the angle between the mirror lines, not the angle itself, and the centre is always where the two lines cross, not some other point, and the result of two reflections in intersecting lines is a rotation, never another reflection.
- (b) (−2, −1) — Method: apply the reflection to the point first, then rotate the image about the given centre, in the order the question states them. Working: reflecting (3, 4) in the line y = 1 keeps x = 3 and puts the image as far below the line as the point is above it: 4 is 3 units above y = 1, so the image is 3 units below, at 1 − 3 = −2 (the same as 2 × 1 − 4 = −2). The reflected point is (3, −2). Rotating (3, −2) by 90° clockwise about (1, 1): subtracting the centre gives 3 − 1 = 2 and −2 − 1 = −3, the clockwise rule swaps and negates these to give −3 and −2, and adding the centre back gives 1 + (−3) = −2 and 1 + (−2) = −1. The final image is (−2, −1). Answer: (−2, −1). Reflect before you rotate, exactly as the design process is described, and rotate about the CENTRE (1, 1) given in the question rather than the origin: either mistake, or reversing the two steps, sends the tile to a different point.
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