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Basic Constructions

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Basic Constructions - Lesson Summary

The tools in a geometry box are:

  • Ruler
  • Compass
  • Divider
  • Set squares
  • Protractor

A ruler is a flat and straight-edged strip, whose one side is graduated into centimetres and the other into inches. A ruler is commonly called a scale. It is the most essential tool in geometry. It is used in all constructions. 

The basic uses of a ruler are:

  • Measuring lengths of line segments
  • Drawing line segments 

A compass has two ends. One end holds a pointer, while the other end holds a pencil. It is also called a pair of compasses.
The basic uses of a compass are:

  • Marking off equal lengths
  • Drawing arcs
  • Drawing circles 

A divider is a tool similar in shape to a compass. It has a pair of pointer ends.
The basic uses of a divider are:

  • Comparing lengths of line segments
  • Helping avoid positioning errors
  • Taking accurate measurements

Set  squares:
The two triangular tools in the geometry box are called set squares. One of the set squares is an isosceles triangle with two angles measuring 45° each. The other set square is a scalene triangle with two angles measuring 30° and 60° each. The two perpendicular sides of either set square are graduated into centimetres. 

The basic uses of set squares are:

  • Drawing perpendicular lines
  • Drawing parallel lines

A semi-circular tool with degrees marked is called a protractor. The centre of the semicircle is called the midpoint of the protractor. This point helps as a reference point for the protractor. The horizontal line is called the base line or the straight edge of the protractor. 

The basic uses of a protractor are:

  • Measuring angles
  • Drawing angles

The important points to be remembered while using the tools for construction are:

  • Draw smooth and thin lines
  • Mark points lightly
  • Maintain tools or instruments with sharp pointers and fine edges
  • Keep two pencils in the box. One is for drawing lines and marking points. The other is for using in the compass

Drawing a line segment of given length:

  • Place the ruler on a drawing paper.
  • Mark point A at the zero of the ruler.
  • Count the divisions in the ruler till you reach the required length.
  • Mark the point as B.
  • Join points A and B.
  • AB is the required line segment of length 10 cm.

Drawing a circle with a given radius:

  • Drawing a circle of radius 5 cm.
  • Set the compass to the desired radius, say 5 cm.
  • Mark point O.
  • Place the pointer end of the compass at point O.
  • Make a complete round with the pencil end of the compass, keeping the pointer at the same position.
  • The required circle of radius 5 cm is obtained.


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