Enlarge a shape with a negative scale factor so the image appears on the opposite side of the centre, upside down. It is Higher only, aimed at grade 6. Free questions with answers are on this page. Worked examples show each line before you try the questions.
A negative scale factor enlarges and rotates 180∘ about the centre: the image is on the opposite side of the centre, and the shape is upside-down relative to the original. Scale factor −2 means twice as far from the centre, other side, twice the size. Count squares from the centre to a vertex, then the same count the other way, times ∣k∣.
Describe a given enlargement with centre, scale factor (with the minus) and, if needed, the word enlargement. The image and object are similar; corresponding sides are parallel if k is positive, and parallel-but-opposite if k is negative (still parallel as lines).
Negative scale factor: the image is on the other side of the centre, and rotated 180∘. Vector from centre × scale factor, then add the centre back. Scale factor −1, centre O, is a half-turn.
Objectives
Find the image of a point under a negative scale factor.
Find the scale factor from a point and its image.
Find the image distance from the origin.
Key points
Draw an enlargement with a negative scale factor from a given centre.
Use vectors from the centre to each vertex, multiply by the scale factor, then measure back from the centre.
Describe a negative enlargement fully: the word 'enlargement', the scale factor and the centre.
Recognise that a negative scale factor between −1 and 0 makes the image smaller and inverted.
Worked examples
Example 1
Enlarge the point (3, 1) with scale factor -2 and centre (0, 0). Find the image.
2 marks
Multiply x-coordinate by -2: 3 × -2 = -6.
Multiply y-coordinate by -2: 1 × -2 = -2.
Combine coordinates into image point: (-6, -2).
Answer: (-6, -2)
Common slips
Slip: Add -2 to x-coordinate: 3 + (-2) = 1. Enlargement multiplies coordinates, not adds.
Slip: Leave y unchanged as 1. Both coordinates must be multiplied by the scale factor.
Slip: Keep signs positive: (6, 2). A negative factor reverses signs through the origin.
Example 2
The enlargement maps (3, 1) to (-6, -2). Find the scale factor.
2 marks
Divide image x by original x: -6 ÷ 3 = -2.
Verify with y-coordinates: -2 ÷ 1 = -2.
State the negative scale factor: -2.
Answer: -2
Common slips
Slip: Subtract coordinates: -6 - 3 = -9. Scale factor is the ratio of coordinates, not difference.
Slip: Divide original by image: 3 ÷ -6 = -0.5. Scale factor is image divided by object.
Slip: State positive factor 2. A change in direction means the factor is negative.
Example 3
The point (3, 1) is enlarged with scale factor -2 and centre (0, 0). Find the distance from the origin to the image. Give your answer as a simplified surd.
3 marks
Calculate object distance to origin: √(3^2 + 1^2) = √10.