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SAI International School

CLASS - VII

Mathematics
CHAPTER-2: FRACTIONS AND DECIMALS
Lesson Notes- 2

SUBTOPIC : ROTATIONAL SYMMETRY

 A figure has line symmetry, if there is a line about which the figure may
be folded so that the two parts of the figure will coincide.
 Regular polygons have equal sides and equal angles. They have
multiple (i.e., more than one) lines of symmetry.
 Each regular polygon has as many lines of symmetry as it has sides.

Regular Regular Equilateral
Regular Polygon Square
hexagon pentagon triangle
Number of lines of 6 5 4 3
symmetry

 Mirror reflection leads to symmetry, under which the left-right orientation
have to be taken care of.
 Rotation turns an object about a fixed point. This fixed point is the
centre of rotation. The angle by which the object rotates is the angle of
rotation.

 A half-turn means rotation by 18001800 ; a quarter-turn means rotation
by 900900 . Rotation may be clockwise or anticlockwise.
 If, after a rotation, an object looks exactly the same, we say that it has a
rotational symmetry.
 In a complete turn (of 36003600 ), the number of times an object looks
exactly the same is called the order of rotational symmetry. The order of
symmetry of a square, for example, is 4 while, for an equilateral triangle,
it is 3.
 Some shapes have only one line of symmetry, like the letter E; some
have only rotational symmetry, like the letter S; and some have both