### 6) Fractional enlargements and finding the centre of enlargement

##### Fractional enlargements

When a shape is enlarged by a **scale factor** between 0 and 1, the image is smaller than the original shape.

The triangle ABC is enlarged by a scale factor of . All the sides of triangle A’B’C’ are one third as long as the sides of the original triangle ABC.

Enlarge the triangle ABC by a scale factor of about the centre of enlargement O.

First, draw ray lines from O to each corner of the triangle.

Next, measure the distance from O to each corner of ABC. Divide the distance by two and plot the points A’ B’ and C’. Alternatively these distances can be shown as vectors. OA = so under a scale factor of , OA’ = .

Finally, join up the points A’ B’ C’.

##### Finding the centre of enlargement

To find the centre of enlargement, draw ray lines from the corners of the image through the corners of the original shape.

Describe the transformation of the triangle RST.

Draw ray lines from the corners of triangle RST through the corners of R’S’T’ until they cross. This is the centre of enlargement.

The triangle has been enlarged by a scale factor of about the centre of enlargement .

##### Negative enlargements

An enlargement with a negative **scale factor** produces an image on the other side of the centre of enlargement. The image appears upside down.

The rectangle ABCD has been enlarged by a scale factor of .

The lengths in rectangle A’B’C’D’ are times as long as rectangle ABCD. The distance from O to A’B’C’D’ is half the distance from O to ABCD.

##### Question 1

Enlarge the quadrilateral EFGH by a scale factor of -½ about the centre of enlargement O

First, draw ray lines from each corner of the quadrilateral to O and extend them

Next, measure the distance from O to each corner of EFGH. Divide the distance by two and plot the points E’ F’ G’ and H’ on the other side of the centre of enlargement. Finally, join up the points E’ F’ G’ and H’

An enlargement by a scale factor of -1 is the same as a rotation of 180°.

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