Class 7 Chapter 12: Symmetry

# Class 7 Chapter 12: Symmetry

Important Concepts and Formulas

1.      A figure is symmetrical about the line which divides the figure into two identical parts.

2.      The line which divides the figure into two identical parts is called the line of symmetry or mirror line.

3.      Regular polygons have as many lines of symmetry as they have sides. For example, equilateral triangle–3, square–4, pentagon–5 and hexagon–6, etc.

4.      When half of a figure is the mirror image of the other, it is said to have reflection symmetry.

5.      Some of the capital letters of the English alphabet are symmetrical such as A, B, C, D, E, H, I, M, O, T, U, V, W, X.

6.      Non-symmetrical shapes are called asymmetrical shapes.

7.      Rotational symmetry is the property that an object has in which it looks the same after some rotation by a partial turn. A complete turn is equal to 360°.

8.      When an object rotates about a fixed point, this fixed point is called the center of rotation.

9.      An object having rotational symmetry looks exactly the same after every rotation through a particular angle and that angle is called the angle of rotation.

10. If x° is the angle of rotation of a shape, then its order of rotational symmetry is 360° ÷ x°.

11. The smallest order of rotational symmetry is 2. The order of rotational symmetry of objects that have no rotational symmetry is considered as 1.