Understanding Elementary Shapes - Revision Notes

 CBSE Class 6 Mathematics

Revision Notes
CHAPTER – 5
Understanding Elementary Shapes

  • The distance between the end points of a line segment is its length.
  • A graduated ruler and the divider are useful to compare lengths of line segments.
  • When a hand of a clock moves from one position to another position we have an example for an angle.
  • One full turn of the hand is 1 revolution.
  • A right angle is ¼ revolution and a straight angle is ½ a revolution.
  • We use a protractor to measure the size of an angle in degrees.
  • The measure of a right angle is 90° and hence that of a straight angle is 180°.
  • An angle is acute if its measure is smaller than that of a right angle and is obtuse if its measure is greater than that of a right angle and less than a straight angle.
  • A reflex angle is larger than a straight angle.
  • Two intersecting lines are perpendicular if the angle between them is 90°.
  • The perpendicular bisector of a line segment is a perpendicular to the line segment that divides it into two equal parts.
  • Triangles can be classified as follows based on their angles:

Nature of angles in the triangle

      Name

Each angle is acute

Acute angled triangle

One angle is a right angle

Right angled triangle

One angle is obtuse

Obtuse angled triangle

  • Triangles can be classified as follows based on the lengths of their sides:

Nature of sides in the triangle

     Name

All the three sides are of unequal length

Scalene triangle

Any two of the sides are of equal length

Isosceles triangle

All the three sides are of equal length

Equilateral triangle

  • Polygons are named based on their sides.

Number of sides

Name of the Polygon

3

Triangle

4

Quadrilateral

5

Pentagon

6

Hexagon

8

Octagon

  • Quadrilaterals are further classified with reference to their properties.

Properties

Name of the Quadrilateral

One pair of parallel sides

Trapezium

Two pairs of parallel sides

Parallelogram

Parallelogram with 4 right angles

Rectangle

Parallelogram with 4 sides of equal length

Rhombus

A rhombus with 4 right angles

Square

  • ·We see around us many three dimensional shapes. Cubes, cuboids, spheres, cylinders, cones, prisms and pyramids are some of them.