year 8 maths

480 Chapter 8 Transformations and congruence
12 When working without a grid or without 90° angles, a protractor and a pair of compasses are
needed to accurately draw images under rotation. This example shows a rotation of 120° about C .
C
a
Copy this triangle with centre of rotation C onto a sheet of paper.
Z′
C
120°
X′
Z
X
Y
Y′
b Construct three circles with centre C and passing through the vertices of the triangle.
c Use a protractor to draw an image after these rotations.
i 120° anticlockwise ii 100° clockwise
13 Make a copy of this diagram and rotate the shape anticlockwise by 135°
about point C . You will need to use a pair of compasses and a protractor,
as shown in Question 12 .
C
ENRICHMENT
– –
14
Finding the centre of rotation
14 Finding the centre of rotation when the angle is known involves the calculation of an angle inside
an isosceles triangle. For the rotation shown, the angle of rotation is 50° . The steps are given as:
i Calculate ∠CAA′ and ∠CA′A .
(2x + 50 = 180, so x = 65.)
ii Draw the angles ∠ AA′C and ∠ A′AC at 65°, using a protractor.
iii Locate the centre of rotation C at the point of intersection of
AC and A′C .
A′
x° x°
A
a On a sheet of paper draw two points, A and A′, about 4 cm apart.
Follow the steps above to locate the centre of rotation if the angle of
rotation is 40° .
50°
b Repeat part a using an angle of rotation of 100° .
c When a shape is rotated and the angle is unknown, there is a special
C
method for accurately pinpointing the centre of rotation. Research this
method and illustrate the procedure using an example.
Cambridge Maths NSW
Stage 4 Year 8 Second edition
ISBN 978-1-108-46627-1 © Palmer et al. 2018
Cambridge University Press
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