Ch 13 helps students visualise 3D shapes: understanding faces, edges, vertices, drawing nets, creating isometric and oblique sketches, and viewing different cross-sections of solids.
Every 3D shape has faces (flat surfaces), edges (where faces meet), and vertices (where edges meet). Euler's formula connects these: F + V − E = 2, valid for all polyhedra.
A net is a 2D pattern that can be folded to form a 3D shape. Different 3D shapes have different nets. Isometric sketches show 3D shapes on dotted paper preserving measurements. Oblique sketches use front face in actual size with the rest at an angle.
A cross-section is the shape you get when you cut a 3D shape with a plane. Different angles of cutting produce different cross-sections. For example, cutting a cylinder horizontally gives a circle; cutting vertically gives a rectangle.
Download: https://ncert.nic.in/textbook/pdf/gemh113.pdf | Complete book: https://ncert.nic.in/textbook/pdf/gemh1ps.zip
Euler's formula states F + V − E = 2, where F = faces, V = vertices, E = edges. This holds for all convex polyhedra (e.g., cubes, prisms, pyramids) but not for curved shapes like cylinders.
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