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Geometric language, 2D and 3D shapes

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Notas de aula

Key Geometric Terms and Notation

  • Point: an exact location, named with a capital letter (e.g., A).
  • Line: a straight path extending infinitely both ways; line segment has two endpoints; ray has one endpoint.
  • Vertex (plural vertices): a corner where two or more edges meet; edge: a line segment where two faces meet.
  • Plane: a flat surface that extends infinitely; a face of a 3D shape lies in a plane.
  • Parallel lines are distinct coplanar lines that never meet. Perpendicular lines meet at 90°, marked with a small square.
  • Congruent shapes have identical size and shape. Similar shapes have corresponding lengths in a constant ratio; scale factor 1 is allowed.
  • Regular polygon: all sides equal and all interior angles equal.

Similar triangles

Similar triangles4 cm3 cm5 cm8 cm6 cm10 cmCorresponding lengths are multiplied by 2.

Properties of 2D Shapes

  • Triangle types: equilateral (3 equal sides, 3 equal angles), isosceles (2 equal sides, 2 equal angles), scalene (no equal sides), right-angled (one 90° angle).
  • Quadrilateral types: square (4 equal sides, 4 right angles), rectangle (opposite sides equal, 4 right angles), parallelogram (opposite sides parallel and equal), rhombus (4 equal sides, opposite angles equal), trapezium (one pair of parallel sides), kite (two pairs of adjacent equal sides).
  • Polygon naming: 3 sides = triangle, 4 = quadrilateral, 5 = pentagon, 6 = hexagon, 8 = octagon.
  • A regular polygon has all sides and angles equal; e.g., regular hexagon has 6 equal sides and 6 equal angles.
  • Interior angle sum of an n-sided polygon = (n − 2) × 180°.
  • Diagonals: lines connecting non-adjacent vertices; a square has 2 diagonals, a pentagon has 5.

Regular hexagon

Regular hexagon4 cm120°

Faces, Edges, and Vertices of 3D Shapes

  • Cube: 6 square faces, 12 edges, 8 vertices.
  • Cuboid: 6 rectangular faces (opposite faces equal), 12 edges, 8 vertices.
  • Triangular prism: 5 faces (2 triangles, 3 rectangles), 9 edges, 6 vertices.
  • Cylinder: 2 circular faces and 1 curved surface; 2 edges (circles), 0 vertices.
  • Cone: 1 circular face, 1 curved surface, 1 vertex (apex), 1 curved edge.
  • Sphere: 1 curved surface, 0 edges, 0 vertices.
  • Square-based pyramid: 5 faces (1 square, 4 triangles), 8 edges, 5 vertices.

Square-based pyramid

Square-based pyramid5 cm4 cmslant height = √(5² + 2²) = 5.39 cm

Prisms and Pyramids

  • A right prism has a constant polygonal cross-section and rectangular lateral faces. The face, edge and vertex formulas below apply to polygonal prisms.
  • Cuboid and cube are special prisms with rectangular cross-sections.
  • A cylinder has a constant circular cross-section, but it is not a polygonal prism.
  • Pyramid: a solid with a polygon base and triangular faces meeting at a common apex; named by its base (e.g., square-based pyramid).
  • A cone has a circular base and curved surface meeting at an apex; it is not a polygon-based pyramid.
  • For a prism, number of faces = (number of sides of cross-section) + 2; edges = 3 × (number of sides); vertices = 2 × (number of sides).
  • For a pyramid with an n-sided base: faces = n + 1, edges = 2n, vertices = n + 1.

Cylinder: flat and curved surfaces

Cylinder: flat and curved surfaces3 cm5 cm

Nets of 3D Shapes

  • A net is a 2D pattern that folds to form a 3D shape.
  • A cube has 11 different nets; each net has 6 squares arranged so they fold without overlap.
  • To check a net: ensure all faces are present and connected, and that folding along edges produces the correct solid.
  • Common nets: cuboid (6 rectangles), triangular prism (2 triangles + 3 rectangles), square-based pyramid (1 square + 4 triangles).
  • Cylinder net: two circles and a rectangle (curved surface unrolled).
  • Cone net: one circle and a sector of a circle.
  • A net must not have flaps that overlap when folded.

A valid cube net

A valid cube netsurface area = 2(9 + 9 + 9) = 54 cm²

Plans and Elevations

  • Plan: the view from directly above (top view).
  • Front elevation: the view from the front.
  • Side elevation: the view from the side (often right side).
  • For a 3D shape made of cubes, draw the outline of each view, showing visible squares.
  • Plan and elevations are 2D drawings that show the shape's dimensions from different directions.
  • Example: a cuboid of length 3, width 2, height 1 has plan 3×2, front elevation 3×1, side elevation 2×1.

Views of a cuboid

Views of a cuboidViewDirectionRectangle / cmPlanFrom above3 × 2Front elevationFrom front3 × 1Side elevationFrom side2 × 1

Common Misconceptions to Avoid

  • A square is a rectangle (it has 4 right angles and opposite sides equal) – don't think they are unrelated.
  • A rhombus with equal sides is not necessarily a square unless all angles are 90°.
  • Here, count flat faces separately from curved surfaces. The circular rims of cylinders and cones are curved edges.
  • When counting edges of a pyramid, remember the base edges plus the slant edges.
  • A net must fold without gaps or overlaps; a misplaced flap makes it invalid.
  • Don't confuse plan (top view) with front elevation (front view).

Typical Exam Questions

  • Name a shape from its properties (e.g., 'I have 4 equal sides and 4 right angles' → square).
  • State the number of faces, edges, and vertices of a given solid (e.g., a triangular prism).
  • Draw the net of a given solid (e.g., a cuboid).
  • Draw the plan and elevations of a 3D shape built from cubes.
  • Identify which of several nets forms a cube (only 11 of many arrangements work).
  • Use correct notation: mark parallel sides with arrows, equal sides with dashes, and right angles with squares.

Slides

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Questões de prática

Prévia grátis — 8 de 62 perguntas. Cadastre-se para ver todas.
  1. 1.Which of the following is the correct notation for a point?

    Easy
    • AA
    • BAB
    • C∠ABC
    • Dline l
  2. 2.How many edges does a triangular prism have?

    Easy
    • A6
    • B9
    • C12
    • D5
  3. 3.A square is a type of rectangle.

    Easy

    True or false?

  4. 4.Which of the following is a regular polygon?

    Medium
    • AEquilateral triangle
    • BNon-square rectangle
    • CNon-equilateral isosceles triangle
    • DNon-rectangular parallelogram
  5. 5.Which of the following are properties of a cube? (select all that apply)

    Medium
    • A6 faces
    • B8 vertices
    • C12 edges
    • DAll faces are rectangles
    • EAll faces are squares
  6. 6.Match each solid to its number of flat faces. Count curved surfaces separately.

    Medium
    • Cube
    • Triangular prism
    • Cylinder
    • 6
    • 5
    • 2
  7. 7.Arrange the following shapes in order of increasing number of sides: pentagon, triangle, hexagon, quadrilateral.

    Medium
    • Triangle
    • Quadrilateral
    • Pentagon
    • Hexagon
  8. 8.Match each solid to the correct number of edges.

    Easy
    • Cube
    • Triangular prism
    • Square-based pyramid
    • Cylinder
    • 12
    • 9
    • 8
    • 2

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