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## 3-D figures

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**Reflectional Symmetry**For 2D figure: If a plane figure can be divided by a line into two identical parts and these parts are mirror images of each other, then we say that the figure has reflectional symmetry. axis of symmetry->**Reflectional Symmetry**For 3D- figure: If a solid is evenly divided into two parts by a plane, the two parts are the mirror images of each other. This solid is said to have the property of reflectional symmetry and the plane is called the plane of reflection.**Cube**How many planes of reflection of a cube? There are totally 9 planes of reflection for a cube**5**4 6 3 1 2 Tetrahedron How many planes of reflection of a tetrahedron? There are totally 6 planes of reflection for a tetrahedron**Rotational symmetry**• If a solid coincides with itself n times (where n 2) when it is rotated one revolution about an axis inside the solid, the solid is said to have n-fold rotational symmetry about the axis, and the axis is called an axis of rotation. • Note: Cube (3D) has rotational symmetry but square (2D) has not**Cube**How many axes rotational symmetry axes of a cube? There are 13 rotational Symmetry axes**Tetrahedron**How many axes rotational symmetry axes of a tetrahedron? There are 7 Rotational symmetry axes**Orthographical Projection**An orthographic projection uses three plane figures to illustrate the shape of a solid. These plane figures show the front view, the side view and the top view of the solid. Front view - viewing the object from the front. Side view - viewing the object from the side. Top view - viewing the object from the top.**Front view :**Side view : Top view : Pyramid**Nets of solids**A net is a plane figure that can be folded into a solid.**Euler Formula**V: number of vertices = 4 E: number of edges = 6 F: number of faces = 4 V- E + F = 2