ANGLES AND ITS TYPE

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Definition of angle: An angle is the union of two non-collinear rays with a common initial point.

  • The two  rays forming an angle are called the "arms" of the angle and the common initial point is called the "vertex" of the angle.

Angle measure axiom: Every angle has a measure. The unit of angle measure is a standard angle called a "degree".

Types of angles:
  • Acute angle: An angle whose measure is less than 90 degree is called an acute angle.
  • Obtuse angle: An angle whose measure is more than 90 degree but less than 180 degree is called an obtuse angle.
  • Right angle: An angle whose measure is 90 degree  is called a right angle.
  • Straight angle: An angle measuring 180 degree is called Straight angle.
  • Reflex angle: An angle whose measure is more than 180 degree is called reflex angle.
  • Complete angle: An angle measuring 360 degree is called complete angle.
  • Complementary angle: Two angles having their sum equal to 90 degree, are called complementary angle.
  • Supplementary angle: Two angles having their sum equal to 180 degree, are called supplementary angle.
Angle bisector: A ray is called angle bisector if it divides an angle into two equal parts.
Adjacent angles: Two angles are called adjacent angles if 
  • they have same vertex
  • common arm and 
  • uncommon arm on either side of common arm

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