8.46 Chapter 8
To describe a rotation clearly, we have to state:
(i) the centre of rotation,
(ii) the direction of rotation (clockwise or anticlockwise), and
(iii) the angle of rotation.
Take Fig. 3 as an example,
(i) A(3 , 0) is rotated anticlockwise about O through 90c to B(0 , 3).
(ii) A(3 , 0) is rotated clockwise about O through 90c to C(0 , -3).
Fig. 3
y
x
O
A(3,0)
90°
90°
B(0,3)
C(0,-3)
Let's investigate how rotation affects the coordinates of a point.
Take out the transparency film provided in the Activity Kit.
1. Follow the steps below and answer the question.
P(1,3)
-1-2-3 1
1
0 2 3
-1
-2
-3
2
3 y
x
P
-1-2-3 1
1
0 2 3
-1
-2
-3
2
3 y
x
P
-1-2-3 1
1
0 2 3
-1
-2
-3
2
3 y
x
y
x
O
P
y
x
O
P Rotate the film
anticlockwise
about O
through 90°
Place the
film over
Fig. A
1 unit
3 units
1 unit
3 units
Fig. A
Denote the image of P by P1.
Coordinates of P1 = ( , )
2. Further rotate the film on Fig. A anticlockwise about O through 90c.
Denote the image of P1 by P2.
Coordinates of P2 = ( , )
3. Further rotate the film on Fig. A anticlockwise about O through 90c.
Denote the image of P2 by P3.
Coordinates of P3 = ( , )
ActivityActivity
Class 8.6 Class e-Activity
Teachers may also use GeoGebra
to conduct the activity.
-3
-1
3
1
-3
-1