☼ Definitions  &  Review of  Concepts  :

■ Angle  :   When a ray  OA  rotates about the point  O from its initial position  OA to find position OB, we say that  ∠ AOB has been formed.

OA is called the initial side and  OB, the terminal side of  ∠AOB.

■ Positive  &  Negative Angles  :   When  the ray  OA  rotates in anticlockwise direction, a positive angle is formed, and when it moves in clockwise direction, a negative angle is formed.

■ Quadrants  :   Let  OX  and  YO  be two lines at right angles to each other. We call  OX  and  YO  as  x – axis and  y – axis respectively.

These lines divide the plane into 4 parts.

The parts  XOY, YO, O  and  OX  are known as  1^st, 2^nd , 3^rd and  4^th quadrant respectively.

An angle is said to be in a particular quadrant, if the terminal side of  the angle lines in that quadrant.

■ Measurement of  Angles  :   Mainly there are three system for measuring angles.

 ▪ Hexadecimal  system  :   In this system, angle is measured in degrees.

1 right angle  =  90^o,  1^o  = 60′   and    =  60″

 ▪ Centesimal  ( or Decimal )  system    :    In  this  system  angle is measured in grades.

1 right angle  =  100^g,  1^g  =     and    =  100″.

 ▪ Circular Measure  :   In this system, angle is measured in radians.

The angle is subtended at the centre of  a circle by an arc of  length equal to its radius is   1 radian,  written as 1^c.

  =  180^o  =  200^g  =  2 right angles.

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