Page 207 - ICSE Math 7

(b) Order of rotational symmetry = 5
360°
Smallest angle of rotation = = 72°
5 72 ° 72 °
\ Angles of rotation are 72°, 144° (72° + 72°), 72 ° 72 °
72 °
216° (72° + 72° + 72°), 288° (72° + 72° + 72° + 72°)

and 360° (72° + 72° + 72° + 72° + 72°).
Example 6: Complete the table.

Shape Centre of rotation Order of rotation Angle of rotation

Rhombus

Equilateral
Triangle

Regular
Hexagon

Circle

Order of Angle of
Solution: Shape Centre of rotation
rotation rotation
Rhombus Point of intersection of diagonals 2 180°

Equilateral
Triangle Point of intersection of medians 3 120°

Regular Point of intersection of diagonals
Hexagon joining the opposite vertices 6 60°

Circle Centre of circle Infinite Any angle

EXERCISE 18.2

1. Write the coordinates of reflection of the following points in x-axis.
(a) (10, –10) (b) (–8, –7) (c) (7, 6) (d) (–3, 9) (e) (–5, 0)

2. Write the coordinates of reflection of the following points in y-axis.
(a) (0, –5) (b) (7, 10) (c) (–3, 5) (d) (4, –6) (e) (–8, –9)
3. Write the coordinates of reflection of the following points in origin.

(a) (5, 4) (b) (–3, –1) (c) (2, –8) (d) (–12, 9) (e) (0, 0)
4. Draw a square whose vertices are P(2, 2), Q(1, 2), R(1, 1) and S(2, 1). Draw its reflection in x-axis.
5. Draw a DABC with A(–2, 1), B(1, 3) and C(3, 2). Find its reflection in y-axis.
6. Mark points A(5, 4) and B(3, –2). Join A and B and find the image of AB in x-axis.

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