Lines and Angles

 
8.1  Lines and Angles
 
Congruent line segments:
 
Definition

Line segments having the same length.

 
  • A line segment is denoted using capital letters at both ends.
 
Example

We can see that both line segments, \(PQ\) and \(RS\), have the same length.

Thus, \(PQ\) and \(RS\) are congruent.

 
Congruent angles:
 
Definition

Angles having the same size.

 
  • An angle is denoted using the symbol ‘\(\angle\)’ and capital letters at the vertex and at the ends of the two arms of the angle.

 
Example

We can see that,

\(\angle PQR=40^\circ\) or \(\angle RQP=40^\circ\).

Thus, \(\angle PQR\) and \(\angle RQP\) are congruent.

 
Estimate and measure the length of a line segment and the size of an angle:
 
  • An angle that appears more than a right angle has an angle greater than \(90^\circ\).
  • An angle that appears less than a right angle has an angle less than \(90^\circ\).
 

 
  • The size of an angle can be measured more accurately using a protractor.
 
The properties of the angle on a straight line, a reflex angle and the angle of one whole turn:
 

The angle on a straight line

  • The sum of angles on a straight line is \(180^{\circ}\).
 

Reflex angle

  • An angle with a size more than \(180^{\circ}\) and less than \(360^{\circ}\).

 

The angle of one whole turn

  • The sum of angles at a point is \(360^{\circ}\).

 
The properties of complementary angles, supplementary angles and conjugate angles:
 

Complementary angles

  • The sum of the two angles is always \(90^\circ\).
 

Supplementary angles

  • The sum of the two angles is always \(180^{\circ}\).

 

Conjugate angles

  • The sum of the two angles is always \(360^{\circ}\).

 
Perform a geometrical construction:
 

(i) Line segments

  • A section of a straight line with a fixed length.
 
Example

Construct the line segment \(AB\) with a length of \(8\text{ cm}\) using only a pair of compasses and a ruler.

 

(ii) Perpendicular bisectors

  • If line \(AB\) is perpendicular to line segment \(CD\) and divide \(CD\) into two parts of equal length, then line \(AB\) is known as the perpendicular bisector of \(CD\).
 
Example

Construct the perpendicular bisector of line segment \(PQ\) using only a pair of compasses and a ruler.

 

(iii) Perpendicular line to straight line

  • If a line is perpendicular to line \(PQ\), then the line is known as perpendicular line to line \(PQ\).
 
Example

Using only a pair of compasses and a ruler, construct the perpendicular line from point \(M\) to the straight line \(PQ\).

 

(iv) Parallel lines

  • Lines that will never meet even when they are extended.
 
Example

Using only a pair of compasses and a ruler, construct the line that is parallel to \(PQ\) passing through point \(R\).

 
Construct angles and angle bisectors:
 
(i) Constructing an angle of \(60^{\circ}\)
 
Example

Using only a pair of compasses and a ruler, construct line \(PQ\) so that \(\angle PQR=60^\circ\).

 

(ii) Angle Bisectors

  • A line divides an angle into two equal angles.
 
Example

Using only a pair of compasses and a ruler, construct the angle bisector of \(\angle PQR\).

 

 

Lines and Angles

 
8.1  Lines and Angles
 
Congruent line segments:
 
Definition

Line segments having the same length.

 
  • A line segment is denoted using capital letters at both ends.
 
Example

We can see that both line segments, \(PQ\) and \(RS\), have the same length.

Thus, \(PQ\) and \(RS\) are congruent.

 
Congruent angles:
 
Definition

Angles having the same size.

 
  • An angle is denoted using the symbol ‘\(\angle\)’ and capital letters at the vertex and at the ends of the two arms of the angle.

 
Example

We can see that,

\(\angle PQR=40^\circ\) or \(\angle RQP=40^\circ\).

Thus, \(\angle PQR\) and \(\angle RQP\) are congruent.

 
Estimate and measure the length of a line segment and the size of an angle:
 
  • An angle that appears more than a right angle has an angle greater than \(90^\circ\).
  • An angle that appears less than a right angle has an angle less than \(90^\circ\).
 

 
  • The size of an angle can be measured more accurately using a protractor.
 
The properties of the angle on a straight line, a reflex angle and the angle of one whole turn:
 

The angle on a straight line

  • The sum of angles on a straight line is \(180^{\circ}\).
 

Reflex angle

  • An angle with a size more than \(180^{\circ}\) and less than \(360^{\circ}\).

 

The angle of one whole turn

  • The sum of angles at a point is \(360^{\circ}\).

 
The properties of complementary angles, supplementary angles and conjugate angles:
 

Complementary angles

  • The sum of the two angles is always \(90^\circ\).
 

Supplementary angles

  • The sum of the two angles is always \(180^{\circ}\).

 

Conjugate angles

  • The sum of the two angles is always \(360^{\circ}\).

 
Perform a geometrical construction:
 

(i) Line segments

  • A section of a straight line with a fixed length.
 
Example

Construct the line segment \(AB\) with a length of \(8\text{ cm}\) using only a pair of compasses and a ruler.

 

(ii) Perpendicular bisectors

  • If line \(AB\) is perpendicular to line segment \(CD\) and divide \(CD\) into two parts of equal length, then line \(AB\) is known as the perpendicular bisector of \(CD\).
 
Example

Construct the perpendicular bisector of line segment \(PQ\) using only a pair of compasses and a ruler.

 

(iii) Perpendicular line to straight line

  • If a line is perpendicular to line \(PQ\), then the line is known as perpendicular line to line \(PQ\).
 
Example

Using only a pair of compasses and a ruler, construct the perpendicular line from point \(M\) to the straight line \(PQ\).

 

(iv) Parallel lines

  • Lines that will never meet even when they are extended.
 
Example

Using only a pair of compasses and a ruler, construct the line that is parallel to \(PQ\) passing through point \(R\).

 
Construct angles and angle bisectors:
 
(i) Constructing an angle of \(60^{\circ}\)
 
Example

Using only a pair of compasses and a ruler, construct line \(PQ\) so that \(\angle PQR=60^\circ\).

 

(ii) Angle Bisectors

  • A line divides an angle into two equal angles.
 
Example

Using only a pair of compasses and a ruler, construct the angle bisector of \(\angle PQR\).